[LeetCode] 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.."]
]

DFS to solve it!

class Solution {
private:
    vector<vector<string> > ret;
    int a[100];
    bool canUse[100];
public:
    bool check(int y, int n)
    {
        for(int i = 0; i < n; i++)
            if (abs(i - n) == abs(y - a[i]))
                return false;
        return true;
    }
    
    void solve(int dep, int maxDep)
    {
        if (dep == maxDep)
        {
            vector<string> ans;
            for(int i = 0; i < maxDep; i++)
            {
                string s;
                for(int j = 0; j < a[i]; j++)
                    s += '.';
                s += 'Q';
                for(int j = 0; j < maxDep - (a[i] + 1); j++)
                    s += '.';
                ans.push_back(s);
            }
            ret.push_back(ans);
            
            return;
        }
        
        for(int i = 0; i < maxDep; i++)
            if (canUse[i] && check(i, dep))
            {
                canUse[i] = false;
                a[dep] = i;
                solve(dep + 1, maxDep);
                canUse[i] = true;             
            }
    }
    
    vector<vector<string> > solveNQueens(int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        ret.clear();
        memset(canUse, true, sizeof(canUse));
        solve(0, n);
        return ret;
    }
};