ClimbingStairs.java

package com.leetcode;

/**
 * Created by jamylu on 2018/1/19.
 * leetcode070
 * 爬台阶
 */
public class ClimbingStairs {

    // 使用递归超时
    public int climbStairs(int n) {
        if (n == 1) {
            return 1;
        } else if (n == 2) {
            return 2;
        } else {
            return climbStairs(n - 1) + climbStairs(n - 2);
        }
    }

    // 动态规划，申请辅助空间O(n)
    public int stairs(int n) {
        int[] dp = new int[n + 2];
        dp[1] = 1;
        dp[2] = 2;
        for (int i = 3; i <= n; i++) {
            dp[i] = dp[i - 1] + dp[i - 2];
        }
        return dp[n];
    }

    // Fibonacci数 O(1)辅助空间
    public int Fibonacci(int n) {
        if (n == 1) {
            return 1;
        }
        if (n == 2) {
            return 2;
        }
        int first = 1;
        int second = 2;
        int current = 0;
        for (int i = 3; i <= n; i++) {
            current = first + second;
            first = second;
            second = current;
        }
        return current;
    }
}