70. Climbing Stairs

原题链接：https://leetcode.com/problems/climbing-stairs/description/
实现如下：

/**
 * Created by clearbug on 2018/2/26.
 */
public class Solution {

    public static void main(String[] args) {
        Solution s = new Solution();
        System.out.println(s.climbStairs1(1));
        System.out.println(s.climbStairs1(2));
        System.out.println(s.climbStairs1(3));
        System.out.println(s.climbStairs1(4));
        System.out.println(s.climbStairs1(5));
        System.out.println(s.climbStairs1(44));
        System.out.println(s.climbStairs2(1));
        System.out.println(s.climbStairs2(2));
        System.out.println(s.climbStairs2(3));
        System.out.println(s.climbStairs2(4));
        System.out.println(s.climbStairs2(5));
        System.out.println(s.climbStairs2(44));
        System.out.println(s.climbStairs3(1));
        System.out.println(s.climbStairs3(2));
        System.out.println(s.climbStairs3(3));
        System.out.println(s.climbStairs3(4));
        System.out.println(s.climbStairs3(5));
        System.out.println(s.climbStairs3(44));
        System.out.println(s.climbStairs4(1));
        System.out.println(s.climbStairs4(2));
        System.out.println(s.climbStairs4(3));
        System.out.println(s.climbStairs4(4));
        System.out.println(s.climbStairs4(5));
        System.out.println(s.climbStairs4(44));
    }

    /**
     * 方法一：分解问题 + 递归
     *
     * @param n
     * @return
     */
    public int climbStairs1(int n) {
        if (n == 1) {
            return 1;
        }
        if (n == 2) {
            return 2;
        }

        return climbStairs1(n - 1) + climbStairs1(n - 2);
    }

    /**
     * 方法二：倒置方法一，避免深层递归
     *
     * @param n
     * @return
     */
    public int climbStairs2(int n) {
        if (n == 1) {
            return 1;
        }
        if (n == 2) {
            return 2;
        }
        int n1 = 2, n2 = 1;
        for (int i = 3; i < n; i++) {
            int curr = n1 + n2;
            n2 = n1;
            n1 = curr;
        }
        return n1 + n2;
    }

    /**
     * 官方答案一：使用组合排序，遍历所有走法，暴力却低效
     */
    public int climbStairs3(int n) {
        return climbStairs3Helper(0, n);
    }

    private int climbStairs3Helper(int start, int n) {
        if (start > n) {
            return 0;
        }
        if (start == n) {
            return 1;
        }

        return climbStairs3Helper(start + 1, n) + climbStairs3Helper(start + 2, n);
    }

    /**
     * 官方答案二：在方法一的基础上加上一个数组来记录已经递归过的答案，避免了重复递归的低效
     */
    public int climbStairs4(int n) {
        int[] memo = new int[n];

        return climbStairs4Helper(0, n, memo);
    }

    private int climbStairs4Helper(int start, int n, int[] memo) {
        if (start > n) {
            return 0;
        }
        if (start == n) {
            return 1;
        }

        if (memo[start] > 0) {
            return memo[start];
        }

        memo[start] = climbStairs4Helper(start + 1, n, memo) + climbStairs4Helper(start + 2, n, memo);
        return memo[start];
    }

    // 官方方法三：动态规划，跟我的方法一和方法二类似
    // 官方方法四：斐波那契数列方法，跟我的方法二基本上是一样的
    // 官方方法五和六涉及到斐波那契数列的一个公式推导的问题，由于这一块我目前不熟悉，那这两种方法后续再学习吧！
}

参考

http://www.sohu.com/a/153858619_466939?p=wechat