leetcode-N-Queens

he 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:
    void solve(int n, int num, vector<bool> &RowUsed, vector<bool> &d1, vector<bool> &d2,vector<vector<string> > &r, vector<string> ss)
    {
        if(num == n)r.push_back(ss);
        for(int i = 0; i < n; i++)
        {
            if((!RowUsed[i])&&(!d1[i+num])&&(!d2[i-num+n-1]))
            {
                RowUsed[i] = true;
                d1[i+num] = true;
                d2[i-num+n-1] = true;
                ss[num][i] = 'Q';
                solve(n,num+1,RowUsed,d1,d2,r,ss);
                RowUsed[i] = false;
                d1[i+num] = false;
                d2[i-num+n-1] = false;
                ss[num][i] = '.';
            }
        }
        return;
    }
    
    vector<vector<string> > solveNQueens(int n) {
        vector<bool> RowUsed(n,false);
        vector<bool> diagonal1(2*n-1,false);
        vector<bool> diagonal2(2*n-1,false);
        string s(n,'.');
        vector<string> ss(n,s);
        vector<vector<string> > ret;
        
        if(n < 1)return ret;
        solve(n,0,RowUsed,diagonal1,diagonal2,ret,ss);
        return ret;
    }
};