LeetCode: N-Queens II [051]

class Solution {
public:

    bool isValid(vector<string>matrix, int x, int y, int n){
        //所在列
        int i=0, j=y;
        for(;i<n;i++){
            if(i!=x&&matrix[i][j]=='Q')return false;
        }
        //指向左上角的斜线
        i=x-1;
        j=y-1;
        while(i>=0&&j>=0){
            if(matrix[i][j]=='Q')return false;
            i--;j--;
        }
        //指向右上角的斜线
        i=x-1;
        j=y+1;
        while(i>=0&&j<n){
            if(matrix[i][j]=='Q')return false;
            i--;j++;
        }
        return true;
    }
    void dfs(int&result, vector<string>matrix, int lineNo, int n){
        if(lineNo>n){
            result++;
            return;
        }
        for(int i=0; i<n; i++){
            matrix[lineNo-1][i]='Q';
            if(isValid(matrix, lineNo-1, i, n)){
                dfs(result, matrix, lineNo+1, n);
            }
            matrix[lineNo-1][i]='.';
        }
    }
    int totalNQueens(int n) {
        int result=0;
        vector<string>matrix(n, string(n,'.'));
        dfs(result, matrix, 1, n);
        return result;
    }
};