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.

Example:

Input: 4
Output: [
 [".Q..",  // Solution 1
  "...Q",
  "Q...",
  "..Q."],

 ["..Q.",  // Solution 2
  "Q...",
  "...Q",
  ".Q.."]
]
Explanation: There exist two distinct solutions to the 4-queens puzzle as shown above.

 

代码：

class Solution {
public:
    bool isValid(int n, int i, int j, vector<int>& record, vector<string>& cur) {
		if(record[j])
			return false;
		for(int ii = i, jj = j; ii >= 0 && jj >= 0; ii--, jj--)
			if(cur[ii][jj] == 'Q')
				return false;
		for(int ii = i, jj = j; ii >= 0 && jj < n; ii--, jj++)
			if(cur[ii][jj] == 'Q')
				return false;
		return true;
	}
	void backSolver(int rest_n, int n, vector<string> cur, vector<int> record, vector<vector<string>>& res) {
		if(rest_n == n)
		{
			res.push_back(cur);
			return;
		}
		for(int i = 0; i<n; i++)
		{
			if(isValid(n, rest_n, i, record, cur))
			{
				cur[rest_n][i] = 'Q';
				record[i] = 1;
				backSolver(rest_n + 1, n, cur, record, res);
				cur[rest_n][i] = '.';
				record[i] = 0;
			}
		}
		return;
	}
	vector<vector<string>> solveNQueens(int n) {
        vector<vector<string>> res;
		vector<string> cur;
		string cur_line = "";
		for(int i = 0; i<n; i++)
			cur_line += ".";
		for(int i = 0; i<n; i++)
			cur.push_back(cur_line);
		vector<int> record(n+1, 0);
		backSolver(0, n, cur, record, res);
		return res;
    }
};