LeetCode 70. Climbing Stairs

70. Climbing Stairs

Difficulty:: Easy

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

Solution

Language: Java

class Solution {
    public int climbStairs(int n) {
        int result = 0;
        int a = 0, b = 1;
        for (int i = 0; i < n; i++) {
            result = a + b;
            a = b;
            b = result;
        }
        return result;
    }
}

class Solution {
    public int climbStairs(int n) {
        if (n <= 0) {
            return 1;
        }
        int[] dp = new int[n + 1];
        dp[0] = 1;
        dp[1] = 1;
        for (int i = 2; i <= n; i++) {
            dp[i] = dp[i - 1] + dp[i - 2];
        }
        return dp[n];
    }
}