52. N-Queens II

Description

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.."]
]

Tags: DFS, 位运算

题意

给一个整数n, 计算在n矩阵格子中能够放Queen的个数

题解

思路1

直接使用三个切片,分别存储列,撇,捺的方式,比较占用空间

思路2

直接使用一个切片,通过该切片元素直接数据换算判断

思路3

使用位运算

// 关键代码
func dfs(n int, row int, cols int, pie int, na int) {
    if row >= n {
        count++
        return
    }
    bits := (^(cols | pie | na) & ((1 << uint(n)) - 1)) // 查看当前能够放置元素的位置
    for bits > 0 {
        p := bits & -bits // 获取最低位1
        dfs(n, row+1, cols|p, (pie|p)>>1, (na|p)<<1)
        bits = bits & (bits - 1) // 去掉最低位1
    }
}

参考文档

结语

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