# 0070. Climbing Stairs

<https://leetcode.com/problems/climbing-stairs>

## Description

You are climbing a staircase. It takes `n` steps to reach the top.

Each time you can either climb `1` or `2` steps. In how many distinct ways can you climb to the top?

**Example 1:**

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

**Example 2:**

```
**Input:** n = 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
```

**Constraints:**

* `1 <= n <= 45`

## ac

linear DP, maintain 2 variables: `prev, curr`

```java
class Solution {
    public int climbStairs(int n) {
        // edge case
        if (n <= 1) return n;

        int prev = 1;
        int curr = 2;
        for (int i = 2; i < n; i++) {
            int tmp = curr;
            curr = curr + prev;
            prev = tmp;
        }

        return curr;
    }
}
```