tags: leetcode easy Math Dynamic Programming Memoization

70. 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?

Examples

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:

• $1 \leq n \leq 45$

Code

int climbStairs(int n){
    if(n < 3) return n;
    int preOne = 0, preTwo = 1, result = 2;
    for(int i = 2 ; i < n ; i++){
        preOne = preTwo;
        preTwo = result;
        result = preOne + preTwo;
    }
    return result;
}

Complexity

Space	Time
$O(N)$	$O(N)$

Result

• Runtime : 0 ms, 100%
• Memory : 5.4 MB, 95.97%