Solution1.java

/// Source : https://leetcode.com/problems/climbing-stairs/description/
/// Author : liuyubobobo
/// Time   : 2017-11-18

import java.util.Arrays;

/// Memory Search
/// Time Complexity: O(n)
/// Space Complexity: O(n)
public class Solution1 {

    private int[] memo;

    public int climbStairs(int n) {

        if(n <= 0)
            throw new IllegalArgumentException("n must be greater than zero");

        memo = new int[n+1];
        Arrays.fill(memo, -1);
        return calcWays(n);
    }

    private int calcWays(int n){

        if(n == 0 || n == 1)
            return 1;

        if(memo[n] == -1)
            memo[n] = calcWays(n - 1) + calcWays(n - 2);

        return memo[n];
    }

    public static void main(String[] args) {

        System.out.println((new Solution1()).climbStairs(10));
    }
}