Leetcode-N-Queens

The n-queens puzzle is the problem of placing n queens on ann×n chessboard such that no two queens attack each other.

Given an integer n, return all distinct solutions to the n-queens puzzle.

Each solution contains a distinct board configuration of the n-queens' placement, where'Q' and '.' both indicate a queen and an empty space respectively.

For example,
There exist two distinct solutions to the 4-queens puzzle:

[ [".Q..",  // Solution 1  "...Q",  "Q...",  "..Q."], ["..Q.",  // Solution 2  "Q...",  "...Q",  ".Q.."]]

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class Solution{public:vector<vector<string>> solveNQueens(int n) {vector<vector<string>> res;vector<string> nQueens(n, string(n,'.'));solve(res, nQueens, 0, n);return res;}private:void solve(vector<vector<string>> &res, vector<string> &nQueens, int row, int &n) {if (row == n) {res.push_back(nQueens);return;}for (int col = 0; col < n; col++) {if (isValid(nQueens,row,col,n)) {nQueens[row][col] = 'Q';solve(res, nQueens, row + 1, n);nQueens[row][col] = '.';}}}bool isValid(vector<string> &nQueens, int row, int col, int &n) {for (int i = 0; i != row; ++i)if (nQueens[i][col] == 'Q')return false;for (int i = row - 1, j = col - 1; i >= 0 && j >= 0; --i, --j)if (nQueens[i][j] == 'Q')return false;for (int i = row - 1, j = col + 1; i >= 0&&j <= n; --i, ++j)if (nQueens[i][j] == 'Q')return false;return true;}};