70. Climbing Stairs

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.

Solution

public class Solution {
    public int climbStairs(int n) {
                if (n == 1) return 1;
        if (n == 2) return 2;

        int[] results = new int[n + 1];
        results[1] = 1;
        results[2] = 2;

        for (int i = 3; i <= n; i++) {
            results[i] = results[i - 1] + results[i - 2];
        }

        return results[n];
    }
}

//method 2
public class Solution {
    public int climbStairs(int n) {
       if (n == 1) return 1;
        if (n == 2) return 2;
        int first=1;
        int second=2;
        int third=0;
        for (int i = 3; i <= n; i++) {
           third = first + second;
           first=second;
           second=third;
        }

        return third;
    }
}
// recursion

public class Solution {
    public int climbStairs(int n) {
        if(n==0){
        return 1;
        }
        if(n<0){
        return 0;
        }
        int path1=climbStairs(n-1);
        int path2=climbStairs(n-2);
        return path1+path2;
    }
}