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Commit 0d439d4

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31\. Next Permutation
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Medium
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Implement **next permutation**, which rearranges numbers into the lexicographically next greater permutation of numbers.
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If such an arrangement is not possible, it must rearrange it as the lowest possible order (i.e., sorted in ascending order).
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The replacement must be **[in place](http://en.wikipedia.org/wiki/In-place_algorithm)** and use only constant extra memory.
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**Example 1:**
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**Input:** nums = \[1,2,3\]
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**Output:** \[1,3,2\]
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**Example 2:**
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**Input:** nums = \[3,2,1\]
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**Output:** \[1,2,3\]
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**Example 3:**
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**Input:** nums = \[1,1,5\]
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**Output:** \[1,5,1\]
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**Example 4:**
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**Input:** nums = \[1\]
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**Output:** \[1\]
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**Constraints:**
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* `1 <= nums.length <= 100`
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* `0 <= nums[i] <= 100`
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32\. Longest Valid Parentheses
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Hard
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Given a string containing just the characters `'('` and `')'`, find the length of the longest valid (well-formed) parentheses substring.
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**Example 1:**
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**Input:** s = "(()"
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**Output:** 2
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**Explanation:** The longest valid parentheses substring is "()".
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**Example 2:**
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**Input:** s = ")()())"
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**Output:** 4
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**Explanation:** The longest valid parentheses substring is "()()".
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**Example 3:**
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**Input:** s = ""
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**Output:** 0
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**Constraints:**
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* `0 <= s.length <= 3 * 104`
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* `s[i]` is `'('`, or `')'`.
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33\. Search in Rotated Sorted Array
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Medium
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There is an integer array `nums` sorted in ascending order (with **distinct** values).
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Prior to being passed to your function, `nums` is **possibly rotated** at an unknown pivot index `k` (`1 <= k < nums.length`) such that the resulting array is `[nums[k], nums[k+1], ..., nums[n-1], nums[0], nums[1], ..., nums[k-1]]` (**0-indexed**). For example, `[0,1,2,4,5,6,7]` might be rotated at pivot index `3` and become `[4,5,6,7,0,1,2]`.
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Given the array `nums` **after** the possible rotation and an integer `target`, return _the index of_ `target` _if it is in_ `nums`_, or_ `-1` _if it is not in_ `nums`.
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You must write an algorithm with `O(log n)` runtime complexity.
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**Example 1:**
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**Input:** nums = \[4,5,6,7,0,1,2\], target = 0
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**Output:** 4
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**Example 2:**
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**Input:** nums = \[4,5,6,7,0,1,2\], target = 3
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**Output:** -1
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**Example 3:**
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**Input:** nums = \[1\], target = 0
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**Output:** -1
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**Constraints:**
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* `1 <= nums.length <= 5000`
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* `-104 <= nums[i] <= 104`
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* All values of `nums` are **unique**.
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* `nums` is an ascending array that is possibly rotated.
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* `-104 <= target <= 104`
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34\. Find First and Last Position of Element in Sorted Array
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Medium
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Given an array of integers `nums` sorted in non-decreasing order, find the starting and ending position of a given `target` value.
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If `target` is not found in the array, return `[-1, -1]`.
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You must write an algorithm with `O(log n)` runtime complexity.
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**Example 1:**
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**Input:** nums = \[5,7,7,8,8,10\], target = 8
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**Output:** \[3,4\]
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**Example 2:**
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**Input:** nums = \[5,7,7,8,8,10\], target = 6
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**Output:** \[-1,-1\]
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**Example 3:**
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**Input:** nums = \[\], target = 0
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**Output:** \[-1,-1\]
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**Constraints:**
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* `0 <= nums.length <= 105`
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* `-109 <= nums[i] <= 109`
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* `nums` is a non-decreasing array.
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* `-109 <= target <= 109`
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35\. Search Insert Position
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Easy
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Given a sorted array of distinct integers and a target value, return the index if the target is found. If not, return the index where it would be if it were inserted in order.
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You must write an algorithm with `O(log n)` runtime complexity.
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**Example 1:**
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**Input:** nums = \[1,3,5,6\], target = 5
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**Output:** 2
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**Example 2:**
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**Input:** nums = \[1,3,5,6\], target = 2
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**Output:** 1
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**Example 3:**
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**Input:** nums = \[1,3,5,6\], target = 7
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**Output:** 4
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**Example 4:**
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**Input:** nums = \[1,3,5,6\], target = 0
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**Output:** 0
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**Example 5:**
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**Input:** nums = \[1\], target = 0
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**Output:** 0
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**Constraints:**
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* `1 <= nums.length <= 104`
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* `-104 <= nums[i] <= 104`
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* `nums` contains **distinct** values sorted in **ascending** order.
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* `-104 <= target <= 104`
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36\. Valid Sudoku
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Medium
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Determine if a `9 x 9` Sudoku board is valid. Only the filled cells need to be validated **according to the following rules**:
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1. Each row must contain the digits `1-9` without repetition.
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2. Each column must contain the digits `1-9` without repetition.
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3. Each of the nine `3 x 3` sub-boxes of the grid must contain the digits `1-9` without repetition.
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**Note:**
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* A Sudoku board (partially filled) could be valid but is not necessarily solvable.
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* Only the filled cells need to be validated according to the mentioned rules.
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**Example 1:**
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![](https://upload.wikimedia.org/wikipedia/commons/thumb/f/ff/Sudoku-by-L2G-20050714.svg/250px-Sudoku-by-L2G-20050714.svg.png)
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**Input:**
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board =
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[["5","3",".",".","7",".",".",".","."]
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,["6",".",".","1","9","5",".",".","."]
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,[".","9","8",".",".",".",".","6","."]
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,["8",".",".",".","6",".",".",".","3"]
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,["4",".",".","8",".","3",".",".","1"]
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,["7",".",".",".","2",".",".",".","6"]
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,[".","6",".",".",".",".","2","8","."]
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,[".",".",".","4","1","9",".",".","5"]
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,[".",".",".",".","8",".",".","7","9"]]
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**Output:** true
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**Example 2:**
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**Input:**
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board =
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[["8","3",".",".","7",".",".",".","."]
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,["6",".",".","1","9","5",".",".","."]
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,[".","9","8",".",".",".",".","6","."]
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,["8",".",".",".","6",".",".",".","3"]
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,["4",".",".","8",".","3",".",".","1"]
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,["7",".",".",".","2",".",".",".","6"]
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,[".","6",".",".",".",".","2","8","."]
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,[".",".",".","4","1","9",".",".","5"]
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,[".",".",".",".","8",".",".","7","9"]]
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**Output:** false
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**Explanation:** Same as Example 1, except with the **5** in the top left corner being modified to **8**. Since there are two 8's in the top left 3x3 sub-box, it is invalid.
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**Constraints:**
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* `board.length == 9`
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* `board[i].length == 9`
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* `board[i][j]` is a digit `1-9` or `'.'`.
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37\. Sudoku Solver
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Hard
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Write a program to solve a Sudoku puzzle by filling the empty cells.
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A sudoku solution must satisfy **all of the following rules**:
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1. Each of the digits `1-9` must occur exactly once in each row.
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2. Each of the digits `1-9` must occur exactly once in each column.
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3. Each of the digits `1-9` must occur exactly once in each of the 9 `3x3` sub-boxes of the grid.
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The `'.'` character indicates empty cells.
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**Example 1:**
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![](https://upload.wikimedia.org/wikipedia/commons/thumb/f/ff/Sudoku-by-L2G-20050714.svg/250px-Sudoku-by-L2G-20050714.svg.png)
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**Input:**
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board = [["5","3",".",".","7",".",".",".","."],
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["6",".",".","1","9","5",".",".","."],
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[".","9","8",".",".",".",".","6","."],
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["8",".",".",".","6",".",".",".","3"],
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["4",".",".","8",".","3",".",".","1"],
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["7",".",".",".","2",".",".",".","6"],
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[".","6",".",".",".",".","2","8","."],
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[".",".",".","4","1","9",".",".","5"],
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[".",".",".",".","8",".",".","7","9"]]
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**Output:**
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[["5","3","4","6","7","8","9","1","2"],
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["6","7","2","1","9","5","3","4","8"],
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["1","9","8","3","4","2","5","6","7"],
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["8","5","9","7","6","1","4","2","3"],
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["4","2","6","8","5","3","7","9","1"],
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["7","1","3","9","2","4","8","5","6"],
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["9","6","1","5","3","7","2","8","4"],
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["2","8","7","4","1","9","6","3","5"],
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["3","4","5","2","8","6","1","7","9"]]
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**Explanation:** The input board is shown above and the only valid solution is shown below:
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![](https://upload.wikimedia.org/wikipedia/commons/thumb/3/31/Sudoku-by-L2G-20050714_solution.svg/250px-Sudoku-by-L2G-20050714_solution.svg.png)
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**Constraints:**
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* `board.length == 9`
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* `board[i].length == 9`
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* `board[i][j]` is a digit or `'.'`.
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* It is **guaranteed** that the input board has only one solution.
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38\. Count and Say
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Medium
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The **count-and-say** sequence is a sequence of digit strings defined by the recursive formula:
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* `countAndSay(1) = "1"`
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* `countAndSay(n)` is the way you would "say" the digit string from `countAndSay(n-1)`, which is then converted into a different digit string.
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To determine how you "say" a digit string, split it into the **minimal** number of groups so that each group is a contiguous section all of the **same character.** Then for each group, say the number of characters, then say the character. To convert the saying into a digit string, replace the counts with a number and concatenate every saying.
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For example, the saying and conversion for digit string `"3322251"`:
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![](https://assets.leetcode.com/uploads/2020/10/23/countandsay.jpg)
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Given a positive integer `n`, return _the_ `nth` _term of the **count-and-say** sequence_.
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**Example 1:**
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**Input:** n = 1
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**Output:** "1"
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**Explanation:** This is the base case.
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**Example 2:**
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**Input:** n = 4
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**Output:** "1211"
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**Explanation:**
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countAndSay(1) = "1"
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countAndSay(2) = say "1" = one 1 = "11"
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countAndSay(3) = say "11" = two 1's = "21"
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countAndSay(4) = say "21" = one 2 + one 1 = "12" + "11" = "1211"
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**Constraints:**
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* `1 <= n <= 30`

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