[LC] 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.

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

class Solution {
    public int climbStairs(int n) {
        if (n <= 2) {
            return n;
        }
        int preOne = 2;
        int preTwo = 1;
        int sum = 0;
        for (int i = 3; i <= n; i++) {
            sum = preOne + preTwo;
            preTwo = preOne;
            preOne = sum;
        }
        return sum;
    }
}