leetcode 70. Climbing Stairs

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