LeetCode 52. N-Queens II

52. N-Queens II

Difficulty:: Hard

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.."]
]

Solution

Language: Java

将上一题精简一下

class Solution {
    int count = 0;
    public int totalNQueens(int n) {
        if (n <= 0) {
            return 0;
        }        
        boolean[][] sets = new boolean[3][n * 2 + 1];
        dfsHelper(n, 0, sets);
        return count;
    }
    
    private void dfsHelper(int n, int index, boolean[][] sets) {
        if (index == n) {
            count++;
            return;
        }
        for (int i = 0; i < n; i++) {
            int left = index - i + n;
            int right = index + i;
            if (sets[0][i] || sets[1][left] || sets[2][right]) {
                continue;
            }
            sets[0][i] = true;
            sets[1][left] = true;
            sets[2][right] = true;
            dfsHelper(n, index + 1, sets);
            sets[0][i] = false;
            sets[1][left] = false;
            sets[2][right] = false;
        }
    }
}