题目介绍

题解

法一：返回51题N皇后List的长度
法二：

class Solution {
    private int n, ans;
    private boolean[] onPath, diag1, diag2;

    public int totalNQueens(int n) {
        this.n = n;
        onPath = new boolean[n];
        diag1 = new boolean[n * 2 - 1];
        diag2 = new boolean[n * 2 - 1];
        dfs(0);
        return ans;
    }

    private void dfs(int r) {
        if (r == n) {
            ans++; // 找到一个合法方案
            return;
        }
        for (int c = 0; c < n; c++) {
            int rc = r - c + n - 1;
            if (!onPath[c] && !diag1[r + c] && !diag2[rc]) {
                onPath[c] = diag1[r + c] = diag2[rc] = true;
                dfs(r + 1);
                onPath[c] = diag1[r + c] = diag2[rc] = false; // 恢复现场
            }
        }
    }
}