19.2.8 [LeetCode 52] N-Queens II

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 the number of distinct solutions to the n-queens puzzle.

Example:

Input: 4
Output: 2
Explanation: There are two distinct solutions to the 4-queens puzzle as shown below.
[
 [".Q..",  // Solution 1
  "...Q",
  "Q...",
  "..Q."],

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

class Solution {
public:
    bool valid(vector<int>&a, int x, int y) {
        for (int i = 0; i < x; i++)
            if (y == a[i] || abs(a[i] - y) == x - i)
                return false;
        return true;
    }
    void build(int&ans,vector<int>&now,int row) {
        int n = now.size();
        if (row == n) 
            ans++;
        else {
            for (int i = 0; i < n; i++)
                if (valid(now, row, i)) {
                    now[row] = i;
                    build(ans, now, row + 1);
                }
        }
    }
    int totalNQueens(int n) {
        vector<int>now(n, -1);
        int ans = 0;
        build(ans, now, 0);
        return ans;
    }
};

用&传送参数可以快好多，终于知道上一题为什么比别人慢了