70

70. Climbing Stairs (Easy)

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

Companies:
Amazon, Google, Microsoft, Bloomberg, Expedia, Adobe, Uber, Apple, LinkedIn, Facebook, Walmart Labs, Morgan Stanley, Twilio

Related Topics:
Math, Dynamic Programming, Memoization

Similar Questions:

• Min Cost Climbing Stairs (Easy)
• Fibonacci Number (Easy)
• N-th Tribonacci Number (Easy)

Solution 1. DP

// OJ: https://leetcode.com/problems/climbing-stairs/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(1)
class Solution {
public:
    int climbStairs(int n) {
        int ans = 1, prev = 1;
        while (--n) {
            ans += prev;
            prev = ans - prev;
        }
        return ans;
    }
};

Or

// OJ: https://leetcode.com/problems/climbing-stairs/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(1)
class Solution {
public:
    int climbStairs(int n) {
        int prev = 0, cur = 1;
        while (n--) {
            int next = cur + prev;
            prev = cur;
            cur = next;
        }
        return cur;
    }
};