Leetcode/Adobe,Apple - 70.Climbing Stairs - Fibonacci (DP)

70. Climbing Stairs(DP)
https://leetcode.com/problems/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.
思路： 总结发现每次走的步数都是fibonacci数 
关键词：dynamic programming, fibonacci
Time & Space complexity: O(n) O(1) 

public class Solution {
    public int climbStairs(int n) {
        //n steps has fibonacci(n) ways to get
        if(n<2){ return n;}
        int i = 1;
        int a = 1;
        int b =1;
        int res = 0;
        while(i<n){
            res = a+ b;
            a = b;
            b = res;
            i++;
        }
        return res;        
    }
}