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,

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

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

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

class Solution {
public:
    bool isValid(vector<string>& svec, int i, int j, int n)
    {
        for(int h = 0; h < j; ++h)
        {
            if(svec[i][h] == 'Q')
            {
                return false;
            }
        }
        for(int k = 0; k < i; ++k)
        {
            if(svec[k][j] == 'Q')
            {
                return false;
            }
        }

        int k = i - 1, h = j - 1;
        while(k >= 0 && h >= 0)
        {
            if(svec[k][h] == 'Q')
            {
                return false;
            }
            k--;
            h--;
        }

        k = i + 1;
        h = j + 1;
        while(k < n && h < n)
        {
            if(svec[k][h] == 'Q')
            {
                return false;
            }
            ++k;
            ++h;
        }

        k = i - 1, h = j + 1;
        while(k >= 0 && h < n)
        {
            if(svec[k][h] == 'Q')
            {
                return false;
            }
            --k;
            h++;
        }

        k = i + 1, h = j - 1;
        while(k < n && h >= 0)
        {
            if(svec[k][h] == 'Q')
            {
                return false;
            }
            ++k;
            --h;  
        }
        return true;
    }
    void help(vector<vector<string> >& vsvec, vector<string>& svec, int dep, int n)
    {
        if(dep == n)
        {
            vsvec.push_back(svec);
            return;
        }
        for(int i = 0; i < n; ++i)
        {
            if(isValid(svec, i, dep, n))
            {
                svec[i][dep] = 'Q';
                help(vsvec, svec, dep + 1, n);
                svec[i][dep] = '.';
            }

        }
    }
    vector<vector<string> > solveNQueens(int n)
    {
        vector<vector<string> > vsvec;
        if(n <= 0)
        {
            return vsvec;
        }
        string s = "";
        for(int i = 0; i < n; ++i)
        {
            s += '.';
        }
        vector<string> svec(n, s);
        help(vsvec, svec, 0, n);
        return vsvec;
    }
};