[Leetcode] Climbing Stairs

ou 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?

public class Solution {    public int climbStairs(int n) {        if(n==1) return 1;        if(n==2) return 2;        int sum1=1;        int sum2=2;        int sum3=3;        int i=3;        while(i<=n)        {            sum3=sum1+sum2;            sum1=sum2;            sum2=sum3;            i++;        }        return sum3;    }}