【Lintcode】033.N-Queens II

题目：

Follow up for N-Queens problem.

Now, instead outputting board configurations, return the total number of distinct solutions.

Example

For n=4, there are 2 distinct solutions.

题解：

Solution 1 ()

class Solution {
public:
    void dfs(int &res, int n, vector<int>& pos, int row) {
         if(row >= n) {
             ++res;
             return;
         }
         for(int col=0; col<n; ++col) {
            if (!isValid(pos, row, col)) {
                continue;
            }
            pos[row] = col;
            dfs(res, n, pos, row + 1);
            pos[row] = -1;
         }
    }
    bool isValid(vector<int>& pos, int row, int col) {
        for (int i = 0; i < row; ++i) {
            if (pos[i] == col || abs(row - i) == abs(col - pos[i])) {
                return false; 
            }
        }
        return true;
    }
    int totalNQueens(int n) {
        int res = 0;
        vector<int> pos(n, -1);
        dfs(res, n, pos, 0);
        return res;        
    }
};