LeetCode Top 100 Liked Questions 70. Climbing Stairs (Java版; Easy)

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LeetCode Top 100 Liked Questions 70. Climbing Stairs (Java版; Easy)

题目描述

You are climbing a stair case. It takes n steps to reach to the top.

Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?

Note: Given n will be a positive integer.

Example 1:

Input: 2
Output: 2
Explanation: There are two ways to climb to the top.
1. 1 step + 1 step
2. 2 steps
Example 2:

Input: 3
Output: 3
Explanation: There are three ways to climb to the top.
1. 1 step + 1 step + 1 step
2. 1 step + 2 steps
3. 2 steps + 1 step

第一次做, 斐波那契数列, f(n) = f(n-1) + f(n-2); 空间复杂度O(1); 动态规划版; 核心: 只考虑跳一次, 然后根据不同的跳法进行分类讨论; 只考虑跳一次; 只考虑跳一次; 只考虑跳一次

/*
斐波那契数列:f(n) = f(n-1) + f(n-2)
*/
class Solution {
    public int climbStairs(int n) {
        if(n<1)
            return 0;
        if(n==1)
            return 1;
        if(n==2)
            return 2;
        int a = 1, b = 2, res=0;
        for(int i=3; i<=n; i++){
            res = a + b;
            a = b;
            b = res;
        }
        return res;
    }
}