LeetCode 51 - 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.."]]

My Code

class Solution {public:    bool inline check(vector<string>& str, int i, int j, int n)    {        for (int ii = 0; ii < i; ii++)            if (str[ii][j] == 'Q')                return true;        for (int ii = i - 1, jj = j - 1; ii >= 0 && jj >= 0; ii--, jj--)            if (str[ii][jj] == 'Q')                return true;        for (int ii = i - 1, jj = j + 1; ii >= 0 && jj < n; ii--, jj++)            if (str[ii][jj] == 'Q')                return true;        return false;    }    void doSolve(vector<vector<string> >& strs, vector<string>& str, int i, int                                                                                              j, int n)    {        if (i == n || j == n)            return;        // false -> no clash        bool flag = check(str, i, j, n);        // Clash        if (flag == true)            doSolve(strs, str, i, j + 1, n);        else        {            str[i][j] = 'Q';            if (i == n - 1)                strs.push_back(str);            else                doSolve(strs, str, i + 1, 0, n);            str[i][j] = '.';            doSolve(strs, str, i, j + 1, n);        }    }    vector<vector<string> > solveNQueens(int n) {        vector<string> str(n, string(n, '.'));        vector<vector<string> > strs;        doSolve(strs, str, 0, 0, n);        return strs;    }};

Runtime: 8 ms