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Commit b88a113

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Merge pull request #334 from cheehwatang/add-1814-CountNicePairsInAnArray
Add 1814. Count Nice Pairs in an Array
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‎README.md‎

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<th>Solution</th>
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<th>Topics</th>
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</tr>
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<tr>
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<td align="center">September 13th</td>
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<td>1814. <a href="https://leetcode.com/problems/count-nice-pairs-in-an-array/">Count Nice Pairs in an Array</a></td>
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<td align="center">$\text{\color{Dandelion}Medium}$</td>
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<td align="center">
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/1814.%20Count%20Nice%20Pairs%20in%20an%20Array/CountNicePairsInAnArray.java">Counting & Hash Table</a>
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</td>
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<td align="center">
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<a href="#array">Array</a>,
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<a href="#counting">Counting</a>,
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<a href="#hash-table">Hash Table</a>,
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<a href="#math">Math</a>
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</td>
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</tr>
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<tr>
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<td align="center">September 12th</td>
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<td>2006. <a href="https://leetcode.com/problems/count-number-of-pairs-with-absolute-difference-k/">Count Number of Pairs With Absolute Difference K</a></td>
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<a href="#simulation">Simulation</a>
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</td>
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</tr>
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<td align="center">September 8th</td>
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<td>1155. <a href="https://leetcode.com/problems/number-of-dice-rolls-with-target-sum/">Number of Dice Rolls With Target Sum</a></td>
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<td align="center">$\text{\color{Dandelion}Medium}$</td>
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<td align="center">
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/1155.%20Number%20of%20Dice%20Rolls%20With%20Target%20Sum/NumberOfDiceRollsWithTargetSum_Memoization.java">Dynamic Programming - Memoization</a>
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</td>
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<td align="center">
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<a href="#dynamic-programming">Dynamic Programming</a>
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</td>
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</tr>
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</table>
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</br>
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<hr>
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</td>
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<td></td>
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</tr>
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<tr>
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<td align="center">1814</td>
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<td><a href="https://leetcode.com/problems/count-nice-pairs-in-an-array/">Count Nice Pairs in an Array</a></td>
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<td align="center">
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/1814.%20Count%20Nice%20Pairs%20in%20an%20Array/CountNicePairsInAnArray.java">Java</a>
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</td>
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<td align="center">$\text{\color{Dandelion}Medium}$</td>
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<td align="center">
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<a href="#array">Array</a>,
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<a href="#counting">Counting</a>,
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<a href="#hash-table">Hash Table</a>,
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<a href="#math">Math</a>
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</td>
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<td></td>
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</tr>
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<tr>
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<td align="center">1833</td>
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<td><a href="https://leetcode.com/problems/maximum-ice-cream-bars/">Maximum Ice Cream Bars</a></td>
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</td>
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<td></td>
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</tr>
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<tr>
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<td align="center">1814</td>
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<td><a href="https://leetcode.com/problems/count-nice-pairs-in-an-array/">Count Nice Pairs in an Array</a></td>
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<td align="center">
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/1814.%20Count%20Nice%20Pairs%20in%20an%20Array/CountNicePairsInAnArray.java">Java</a>
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</td>
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<td align="center">$\text{\color{Dandelion}Medium}$</td>
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<td align="center">
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<a href="#array">Array</a>,
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<a href="#counting">Counting</a>,
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<a href="#hash-table">Hash Table</a>,
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<a href="#math">Math</a>
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</td>
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<td></td>
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</tr>
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<tr>
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<td align="center">2006</td>
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<td><a href="https://leetcode.com/problems/count-number-of-pairs-with-absolute-difference-k/">Count Number of Pairs With Absolute Difference K</a></td>
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</td>
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<td></td>
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</tr>
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<tr>
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<td align="center">1814</td>
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<td><a href="https://leetcode.com/problems/count-nice-pairs-in-an-array/">Count Nice Pairs in an Array</a></td>
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<td align="center">
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/1814.%20Count%20Nice%20Pairs%20in%20an%20Array/CountNicePairsInAnArray.java">Java</a>
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</td>
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<td align="center">$\text{\color{Dandelion}Medium}$</td>
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<td align="center">
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<a href="#array">Array</a>,
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<a href="#counting">Counting</a>,
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<a href="#hash-table">Hash Table</a>,
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<a href="#math">Math</a>
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</td>
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<td></td>
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</tr>
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<tr>
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<td align="center">1832</td>
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<td><a href="https://leetcode.com/problems/check-if-the-sentence-is-pangram/">Check if the Sentence is Pangram</a></td>
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/1680.%20Concatenation%20of%20Consecutive%20Binary%20Numbers/ConcatenationOfConsecutiveBinaryNumbers_BitManipulation.java"><em>Bit Manipulation</em></a>
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</td>
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</tr>
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<tr>
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<td align="center">1814</td>
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<td><a href="https://leetcode.com/problems/count-nice-pairs-in-an-array/">Count Nice Pairs in an Array</a></td>
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<td align="center">
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<a href="https://github.com/cheehwatang/leetcode-java/blob/main/solutions/1814.%20Count%20Nice%20Pairs%20in%20an%20Array/CountNicePairsInAnArray.java">Java</a>
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</td>
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<td align="center">$\text{\color{Dandelion}Medium}$</td>
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<td align="center">
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<a href="#array">Array</a>,
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<a href="#counting">Counting</a>,
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<a href="#hash-table">Hash Table</a>,
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<a href="#math">Math</a>
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</td>
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<td></td>
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</tr>
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<tr>
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<td align="center">1979</td>
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<td><a href="https://leetcode.com/problems/find-greatest-common-divisor-of-array/">Find Greatest Common Divisor of Array</a></td>
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package com.cheehwatang.leetcode;
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import java.util.HashMap;
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import java.util.Map;
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// Time Complexity : O(n),
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// where 'n' is the length of 'nums'.
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// We traverse 'nums' to determine and count the relative values.
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//
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// Space Complexity : O(n),
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// where 'n' is the length of 'nums'.
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// We use a Hash Table to count the relative values, with a maximum size of 'n'.
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public class CountNicePairsInAnArray {
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// Approach:
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// First we need to understand the bad pair, i < j and nums[i] + rev(nums[j]) == nums[j] + rev(nums[i]).
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// With the brute force, we can check each and every pair if it is a nice pair,
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// but the algorithm will exceed the time limit.
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// Thus, let's change the equation for the bad pair as follows:
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// nums[i] - rev(nums[i]) == nums[j] - rev(nums[j]).
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// Use a HashMap to record the nums[i] - rev(nums[i]) for all the numbers, and the frequency.
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public int countNicePairs(int[] nums) {
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// Use long type, so as not the keep doing the modulo function, only need to do once at the end.
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long nicePairs = 0;
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Map<Integer, Integer> map = new HashMap<>();
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for (int number : nums) {
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// Calculate the difference (relative value) between the nums[i] and rev(nums[i]).
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int relative = number - reverse(number);
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// If the map already had numbers with the same relative value, then add the frequency as the nice pairs.
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nicePairs += map.getOrDefault(relative, 0);
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// Increase the count in the map.
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map.put(relative, map.getOrDefault(relative, 0) + 1);
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}
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return (int) (nicePairs % (1e9 + 7));
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}
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// Method to reverse the number.
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private int reverse(int number) {
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int reverse = 0;
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while (number > 0) {
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reverse = reverse * 10 + number % 10;
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number /= 10;
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}
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return reverse;
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}
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}

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