hello-algo/codes/java/chapter_dynamic_programming/climbing_stairs_backtrack.java

/**
 * File: climbing_stairs_backtrack.java
 * Created Time: 2023-06-30
 * Author: Krahets (krahets@163.com)
 */

package chapter_dynamic_programming;

import java.util.*;

public class climbing_stairs_backtrack {
    /* 回溯 */
    public static void backtrack(List<Integer> choices, int state, int n, List<Integer> res) {
        // 当爬到第 n 阶时，方案数量加 1
        if (state == n)
            res.set(0, res.get(0) + 1);
        // 遍历所有选择
        for (Integer choice : choices) {
            // 剪枝：不允许越过第 n 阶
            if (state + choice > n)
                break;
            // 尝试：做出选择，更新状态
            backtrack(choices, state + choice, n, res);
            // 回退
        }
    }

    /* 爬楼梯：回溯 */
    public static int climbingStairsBacktrack(int n) {
        List<Integer> choices = Arrays.asList(1, 2); // 可选择向上爬 1 或 2 阶
        int state = 0; // 从第 0 阶开始爬
        List<Integer> res = new ArrayList<>();
        res.add(0); // 使用 res[0] 记录方案数量
        backtrack(choices, state, n, res);
        return res.get(0);
    }

    public static void main(String[] args) {
        int n = 9;

        int res = climbingStairsBacktrack(n);
        System.out.println(String.format("爬 %d 阶楼梯共有 %d 种方案", n, res));
    }
}