hello-algo/codes/kotlin/chapter_backtracking/n_queens.kt

/**
 * File: n_queens.kt
 * Created Time: 2024-01-25
 * Author: curtishd (1023632660@qq.com)
 */

package chapter_backtracking.n_queens

/* 回溯算法：n 皇后 */
fun backtrack(
    row: Int,
    n: Int,
    state: List<MutableList<String>>,
    res: MutableList<List<List<String>>?>,
    cols: BooleanArray,
    diags1: BooleanArray,
    diags2: BooleanArray
) {
    // 当放置完所有行时，记录解
    if (row == n) {
        val copyState: MutableList<List<String>> = ArrayList()
        for (sRow in state) {
            copyState.add(ArrayList(sRow))
        }
        res.add(copyState)
        return
    }
    // 遍历所有列
    for (col in 0..<n) {
        // 计算该格子对应的主对角线和次对角线
        val diag1 = row - col + n - 1
        val diag2 = row + col
        // 剪枝：不允许该格子所在列、主对角线、次对角线上存在皇后
        if (!cols[col] && !diags1[diag1] && !diags2[diag2]) {
            // 尝试：将皇后放置在该格子
            state[row][col] = "Q"
            diags2[diag2] = true
            diags1[diag1] = diags2[diag2]
            cols[col] = diags1[diag1]
            // 放置下一行
            backtrack(row + 1, n, state, res, cols, diags1, diags2)
            // 回退：将该格子恢复为空位
            state[row][col] = "#"
            diags2[diag2] = false
            diags1[diag1] = diags2[diag2]
            cols[col] = diags1[diag1]
        }
    }
}

/* 求解 n 皇后 */
fun nQueens(n: Int): List<List<List<String>>?> {
    // 初始化 n*n 大小的棋盘，其中 'Q' 代表皇后，'#' 代表空位
    val state: MutableList<MutableList<String>> = ArrayList()
    for (i in 0..<n) {
        val row: MutableList<String> = ArrayList()
        for (j in 0..<n) {
            row.add("#")
        }
        state.add(row)
    }
    val cols = BooleanArray(n) // 记录列是否有皇后
    val diags1 = BooleanArray(2 * n - 1) // 记录主对角线上是否有皇后
    val diags2 = BooleanArray(2 * n - 1) // 记录次对角线上是否有皇后
    val res: MutableList<List<List<String>>?> = ArrayList()

    backtrack(0, n, state, res, cols, diags1, diags2)

    return res
}

/* Driver Code */
fun main() {
    val n = 4
    val res: List<List<List<String?>?>?> = nQueens(n)

    println("输入棋盘长宽为 $n")
    println("皇后放置方案共有 ${res.size} 种")
    for (state in res) {
        println("--------------------")
        for (row in state!!) {
            println(row)
        }
    }
}