LeetCode-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?

Solution:

Code:

<span style="font-size:12px;">class Solution {
public:
    int climbStairs(int n) {
        if (n == 0 || n == 1 || n == 2) return n;
        int first = 1, second = 2, third;
        for (int i = 3; i <= n; i++) {
            third = first+second;
            first = second;
            second = third;
        }
        return third;
    }
};</span>