LeetCode（70）——Climbing Stairs

题目：

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.

Example 1:

Input: 2
Output: 2
Explanation: There are two ways to climb to the top.
1. 1 step + 1 step
2. 2 steps

Example 2:

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

AC：

class Solution {
    public int climbStairs(int n) {
        if (n <= 3) {
            return n;
        }

        int first = 2;
        int second = 3;
        int third = first + second;
        
        for (int i = 4; i <= n; i++) {
            third = first + second;
            first = second;
            second = third;
        }
        
        return third;
    }
}