House Robber Solutions in Java

/// Source : https://leetcode.com/problems/house-robber/description/
/// Author : liuyubobobo
/// Time   : 2017-11-19

import java.util.Arrays;

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

    // the max profit for robbing nums[i...n)
    private int[] memo;

    public int rob(int[] nums) {
        memo = new int[nums.length];
        Arrays.fill(memo, -1);
        return tryRob(nums, 0);
    }

    private int tryRob(int[] nums, int index){

        if(index >= nums.length)
            return 0;

        if(memo[index] != -1)
            return memo[index];

        return memo[index] =
                Math.max(tryRob(nums, index + 1),
                         nums[index] + tryRob(nums, index + 2));
    }

    public static void main(String[] args) {

        int nums[] = {2, 1};
        System.out.println((new Solution1()).rob(nums));
    }
}

/// Source : https://leetcode.com/problems/house-robber/description/
/// Author : liuyubobobo
/// Time   : 2017-11-19

/// Dynamic Programming
/// Time Complexity: O(n)
/// Space Complexity: O(n)
public class Solution2 {

    public int rob(int[] nums) {

        int n = nums.length;
        if(n == 0)
            return 0;

        // the max profit for robbing nums[i...n)
        int[] memo = new int[nums.length];
        memo[n - 1] = nums[n - 1];
        for(int i = n - 2 ; i >= 0 ; i --)
            memo[i] = Math.max(memo[i + 1],
                               nums[i] + (i + 2 < n ? memo[i + 2] : 0));

        return memo[0];
    }

    public static void main(String[] args) {

        int nums[] = {2, 1};
        System.out.println((new Solution2()).rob(nums));
    }
}

/// Source : https://leetcode.com/problems/house-robber/description/
/// Author : liuyubobobo
/// Time   : 2017-11-19

/// Dynamic Programming with O(1) Space
/// Time Complexity: O(n)
/// Space Complexity: O(n)
public class Solution3 {

    public int rob(int[] nums) {

        int n = nums.length;
        if(n == 0)
            return 0;

        int curMax = 0, preMax = 0;
        for(int i = n - 1 ; i >= 0 ; i --) {
            int temp = curMax;
            curMax = Math.max(curMax, nums[i] + preMax);
            preMax = temp;
        }
        return curMax;
    }

    public static void main(String[] args) {

        int nums[] = {2, 1};
        System.out.println((new Solution3()).rob(nums));
    }
}

/// Source : https://leetcode.com/problems/house-robber/description/
/// Author : liuyubobobo
/// Time   : 2017-11-19

import java.util.Arrays;

/// Memory Search
/// Another way to define the states
/// Time Complexity: O(n)
/// Space Complexity: O(n)
public class Solution4 {

    // the max profit for robbing nums[0...i]
    private int[] memo;

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

    private int tryRob(int[] nums, int index){

        if(index < 0)
            return 0;

        if(memo[index] != -1)
            return memo[index];

        return memo[index] =
                Math.max(tryRob(nums, index - 1),
                        nums[index] + tryRob(nums, index - 2));
    }

    public static void main(String[] args) {

        int nums[] = {2, 1};
        System.out.println((new Solution4()).rob(nums));
    }
}

/// Source : https://leetcode.com/problems/house-robber/description/
/// Author : liuyubobobo
/// Time   : 2017-11-19

/// Dynamic Programming
/// Another way to define the states
/// Time Complexity: O(n)
/// Space Complexity: O(n)
public class Solution5 {

    public int rob(int[] nums) {

        int n = nums.length;
        if(n == 0)
            return 0;

        // the max profit for robbing nums[0...i]
        int[] memo = new int[nums.length];
        memo[0] = nums[0];
        for(int i = 1 ; i < n ; i ++)
            memo[i] = Math.max(memo[i - 1],
                    nums[i] + (i - 2 >= 0 ? memo[i - 2] : 0));

        return memo[n-1];
    }

    public static void main(String[] args) {

        int nums[] = {2, 1};
        System.out.println((new Solution5()).rob(nums));
    }
}

/// Source : https://leetcode.com/problems/house-robber/description/
/// Author : liuyubobobo
/// Time   : 2017-11-19

/// Dynamic Programming with O(1) space
/// Another way to define the states
/// Time Complexity: O(n)
/// Space Complexity: O(1)
public class Solution6 {

    public int rob(int[] nums) {

        int n = nums.length;
        if(n == 0)
            return 0;

        int curMax = 0, preMax = 0;
        for(int i = 0 ; i < n ; i ++) {
            int temp = curMax;
            curMax = Math.max(curMax, nums[i] + preMax);
            preMax = temp;
        }
        return curMax;
    }

    public static void main(String[] args) {

        int nums[] = {2, 1};
        System.out.println((new Solution6()).rob(nums));
    }
}