70.Climbing Stairs.md

70.Climbing Stairs

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

解析

斐波那契数列 的一种变体，和青蛙跳台阶（剑指offer原题）是一模一样的题。

代码

class Solution {
public:
    int climbStairs(int n) {
        if(n<0)
            return -1;
        if(n==0)
            return 0;
        if(n==1)
            return 1;
        int a=1;
        int b=1;
        for(int i=2;i<=n;i++){
            b=a+b;
            a=b-a;
        }
        return b;
    }
};