LeetCode 0070 - Climbing Stairs

Input: 2
Output:  2
Explanation:  There are two ways to climb to the top.

1. 1 step + 1 step
2. 2 steps

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

class Solution {
public:
    int climbStairs(int n) {
        long long a = 1;
        long long b = 1;
        while(n--)
            a = (b += a) - a;
        return a;
    }
};