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

难度：easy

题目：
爬楼梯用例。要用n 步才能到达顶端.第次可以爬1或2步，有多少种爬上楼顶的方式？
注意：n是正整数。

思路：递归转迭代

Runtime: 2 ms, faster than 100.00% of Java online submissions for Climbing Stairs.
Memory Usage: 25.4 MB, less than 93.69% of Java online submissions for Climbing Stairs.

public class Solution {
    public int climbStairs(int n) {
        if (n <= 1) {
            return 1;
        }
        
        int num1 = 1, num2 = 1, result = num1 + num2;
        for (int i = 1; i < n; i++) {
            result = num1 + num2;
            num1 = num2;
            num2 = result;
        }
        
        return result;
    }
}