LeetCode OJ:N-Queens

N-Queens

 

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 {
    vector<bool> row,col,lslash,rslash;
    vector<int> pos;
    vector<vector<string>> result;
public:
    void dfs(int deep,int n){
        if(deep==n){
            vector<string> ans;
            for(int i = 0; i < n; i++)
            {
                string s;
                for(int j = 0; j < pos[i]; j++)
                    s += '.';
                s += 'Q';
                for(int j = 0; j < n - (pos[i] + 1); j++)
                    s += '.';
                ans.push_back(s);
            }
            result.push_back(ans);
            return;
        }
        for(int i=0;i<n;i++){
            if(!row[deep]&&!col[i]&&!lslash[deep+i]&&!rslash[deep+n-1-i]){
                row[deep]=col[i]=lslash[deep+i]=rslash[deep+n-1-i]=true;
                pos[deep]=i;
                dfs(deep+1,n);
                row[deep]=col[i]=lslash[deep+i]=rslash[deep+n-1-i]=false;
            }
        }
    }
    vector<vector<string> > solveNQueens(int n) {
        row.assign(n,false);//某行是否有皇后了
        col.assign(n,false);//某列是否有皇后了
		lslash.assign(2*n,false);//左斜线是否有皇后了
		rslash.assign(2*n,false);//右斜线是否有皇后了
        pos.assign(n,0);
        dfs(0,n);
        return result;
    }
};