LeetCode N-Queens

题目：

The n-queens puzzle is the problem of placing n queens on an n×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.."]]

class Solution {public:vector<vector<string> > solveNQueens(int n) {res.clear();memset(used, false, sizeof(used));dfs(0, n);return res;}private:vector<vector<string> > res;// a[i]表示第i行的a[i]列有Queenint a[100];int used[100];void dfs(int dep, int maxDep) {if (dep == maxDep) {vector<string> ans;for (int i = 0; i < maxDep; i++) {string s;int j;for (j = 0; j < a[i]; j++)s += '.';s += 'Q';j++;while (j != maxDep) {s += '.';j++;}ans.push_back(s);}res.push_back(ans);return;}for (int y = 0; y < maxDep; y++) {if (check(dep, y) && !used[y]) {used[y] = true;a[dep] = y;dfs(dep + 1, maxDep);used[y] = false;}}}bool check(int x, int y) {for (int i = 0; i < x; i++) {if (abs(i-x) == abs(y-a[i]))return false;}return true;}};