[DP] Rob house.java

/**
You are a professional robber planning to rob houses along a street. Each house has a certain amount of money stashed, the only constraint stopping you from robbing each of them is that adjacent houses have security system connected and it will automatically contact the police if two adjacent houses were broken into on the same night.

Given a list of non-negative integers representing the amount of money of each house, determine the maximum amount of money you can rob tonight without alerting the police.

Example 1:

Input: [1,2,3,1]
Output: 4
Explanation: Rob house 1 (money = 1) and then rob house 3 (money = 3).
             Total amount you can rob = 1 + 3 = 4.

Example 2:

Input: [2,7,9,3,1]
Output: 12
Explanation: Rob house 1 (money = 2), rob house 3 (money = 9) and rob house 5 (money = 1).
             Total amount you can rob = 2 + 9 + 1 = 12.


**/

class Solution {
    public int rob(int[] nums) {
        if(nums.length == 0){
            return 0;
        }
        else if(nums.length == 1){
            return nums[0];
        }
        
        int[] memo = new int[nums.length];
        memo[0] = nums[0]; //take current house
        memo[1] = Math.max(nums[0],nums[1]); //Dont take current house, so can take prev 
        //You have to do math.max because if the scenario of [2,1,1,2] ans is 4 but you will return 3 if you dont do it.
        
        for(int i = 2; i < nums.length; i++){
            memo[i] = Math.max(nums[i] + memo[i-2], memo[i-1]); //take current house + prev prev or dont take current house, meaning use prev. 
        }
        return memo[nums.length -1];
    }
}


int[] memo;
public int rob(int[] nums) {
    memo = new int[nums.length + 1];
    Arrays.fill(memo, -1);
    return rob(nums, nums.length - 1);
}

private int rob(int[] nums, int i) {
    if (i < 0) {
        return 0;
    }
    if (memo[i] >= 0) {
        return memo[i];
    }
    int result = Math.max(rob(nums, i - 2) + nums[i], rob(nums, i - 1));
    memo[i] = result;
    return result;
}