[leetcode-in-go] 0051-N-Queens

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 all distinct solutions to the n-queens puzzle.

Each solution contains a distinct board configuration of the n-queens’ placement, where ‘Q’ and ‘.’ both indicate a queen and an empty space respectively.

Example:

Input: 4
Output: [
[".Q…", // Solution 1
“…Q”,
“Q…”,
“…Q.”],

["…Q.", // Solution 2
“Q…”,
“…Q”,
“.Q…”]
]
Explanation: There exist two distinct solutions to the 4-queens puzzle as shown above.

解题思路

• 回溯法

func available(current []int, index int) bool {
	n := len(current)
	for i, v := range current {
		// 同一列
		if index == v {
			return false
		}

		// 对角线
		if (n-i) == index-v || (n-i) == v-index {
			return false
		}
	}

	return true
}

func core(current []int, n int, result *[][]string) {
	if len(current) == n {
		cur := make([]string, n)
		for i, v := range current {
			buf := bytes.Buffer{}
			for j := 0; j < n; j++ {
				if j != v {
					buf.WriteRune('.')
				} else {
					buf.WriteRune('Q')
				}
			}
			cur[i] = buf.String()
		}
		*result = append(*result, cur)
	}

	for i := 0; i < n; i++ {
		if available(current, i) {
			core(append(current, i), n, result)
		}
	}
}

func solveNQueens(n int) [][]string {
	result := [][]string{}
	if n == 0 {
		return result
	}
	core([]int{}, n, &result)
	return result
}