Leetcode 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 

answer ：

a. recursive （ Overtime ）

class Solution {
    public int climbStairs(int n) {
        if(n < 0)
            return 0;
        if(n == 0 ||n == 1)
            return 1;
        return climbStairs(n - 1) + climbStairs(n - 2);
    }
}

 b. Save results

class Solution {
    public int[] nums = new int[1000];
    public int climbStairs(int n) {
        if(n < 0)
            return 0;
        if(n == 0 ||n == 1)
            return 1;
        if(nums[n] > 0)
            return nums[n];
        nums[n] = climbStairs(n - 1) + climbStairs(n - 2);
        return nums[n];
    }
}