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N-Queens

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	`1`	`+'''Leetcode- https://leetcode.com/problems/n-queens/ '''`
	`2`	`+'''`
	`3`	`+The n-queens puzzle is the problem of placing n queens on an n x n chessboard such that no two queens attack each other.`
	`4`	`+`
	`5`	`+Given an integer n, return all distinct solutions to the n-queens puzzle. You may return the answer in any order.`
	`6`	`+`
	`7`	`+Each solution contains a distinct board configuration of the n-queens' placement, where 'Q' and '.' both indicate a queen and an empty space, respectively.`
	`8`	`+`
	`9`	`+Example 1:`
	`10`	`+`
	`11`	`+Input: n = 4`
	`12`	`+Output: [[".Q..","...Q","Q...","..Q."],["..Q.","Q...","...Q",".Q.."]]`
	`13`	`+'''`
	`14`	`+`
	`15`	`+`
	`16`	`+def solveNQueens(n):`
	`17`	`+`
	`18`	`+ cols = set()`
	`19`	`+ pos = set()`
	`20`	`+ neg = set()`
	`21`	`+`
	`22`	`+ res = []`
	`23`	`+ board = [["."] * n for i in range(n)]`
	`24`	`+`
	`25`	`+ def backtrack(r):`
	`26`	`+ if r == n:`
	`27`	`+ copy = ["".join(row) for row in board]`
	`28`	`+ res.append(copy)`
	`29`	`+ return`
	`30`	`+`
	`31`	`+ for c in range(n):`
	`32`	`+ if c in cols or (r + c) in pos or (r - c) in neg:`
	`33`	`+ continue`
	`34`	`+`
	`35`	`+ cols.add(c)`
	`36`	`+ pos.add(r+c)`
	`37`	`+ neg.add(r-c)`
	`38`	`+ board[r][c] = "Q"`
	`39`	`+`
	`40`	`+ backtrack(r+1)`
	`41`	`+`
	`42`	`+ cols.remove(c)`
	`43`	`+ pos.remove(r+c)`
	`44`	`+ neg.remove(r-c)`
	`45`	`+ board[r][c] = "."`
	`46`	`+`
	`47`	`+ backtrack(0)`
	`48`	`+`
	`49`	`+ return res`
	`50`	`+`
	`51`	`+#T:O(n!)`
	`52`	`+#S:O(N^2)`

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`@@ -20,5 +20,6 @@ Check the notes for the explaination - [Notes](https://stingy-shallot-4ea.notion`
`20`	`20`	`- [x] [Palindrome Partitioning](Backtracking/131-Palindrome-Partitioning.py)`
`21`	`21`	`- [x] [Letter Combinations of a Phone Number](Backtracking/17-Letter-Combinations-of-a-Phone-Number.py)`
`22`	`22`	`- [x] [Generalized Abbreviation](Backtracking/320-Generalized-Abbreviation.py)`
	`23`	`+ - [x] [N-Queens](Backtracking/51-N-Queens.py)`
`23`	`24`
`24`	`25`

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