in LintCode Dynamic Programming ~ read.

Climbing Stairs

LintCode-111.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?

Example

Given an example n=3 , 1+1+1=2+1=1+2=3

return 3

public class Solution {  
    /**
     * @param n: An integer
     * @return: An integer
     */
    public int climbStairs(int n) {
        if (n <= 1) {
            return 1;
        }

        int[] sum = new int[n + 1];
        sum[0] = 0;
        sum[1] = 1;
        sum[2] = 2;

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

        return sum[n];
    }
}

Hope this helps,
Michael