leetcode 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.

public class Solution {
public int climbStairs(int n) {
int a = 1, b = 1;
while(--n>0){
a = (b+=a) - a;
}
return b;
}
}