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 public class Solution { public int climbStairs(int n) { int a=1; int b=2; if(n==1)return a; if(n==2)return b; for(int i=3;i<=n;i++){ int index=a; a=b; b=index+a; } return b; } } class Solution { public int climbStairs(int n) { int[] memory = new int[n+1]; return climb(n, memory); } public int climb(int n, int[] memory) { if(n <= 2) { return n; } if(memory[n] != 0) { return memory[n]; } memory[n] = climb(n-1, memory) + climb(n-2, memory); return memory[n]; } } class Solution { public int climbStairs(int n) { if (n <= 0) return 0; else if (n == 1) return 1; else if (n == 2) return 2; int[] arr = new int[n + 1]; arr[1] = 1; arr[2] = 2; for (int i = 3; i < n + 1; i++) { arr[i] = arr[i -1] + arr[i - 2]; } return arr[n]; } }