Java - Recursive DP with short explanation

  • 1

    Recursive solution with memorization. In the statement is given that we can't use two adjacent nodes. So, we have two variants:

    1. to use current node then jump to children's of children.
    2. to not use current node but use answers from children.
    public class Solution {
        HashMap<TreeNode, Integer> memo = new HashMap<TreeNode, Integer>();
        public int rob(TreeNode root) {
            if (root==null) return 0;
            return calculate(root);
        private int calculate(TreeNode node) {
            if (node==null) return 0;
            if (memo.containsKey(node)) return memo.get(node);
            int use = node.val;
            if (node.left!=null && node.left.left!=null) use += calculate(node.left.left);
            if (node.left!=null && node.left.right!=null) use += calculate(node.left.right);
            if (node.right!=null && node.right.left!=null) use += calculate(node.right.left);
            if (node.right!=null && node.right.right!=null) use += calculate(node.right.right);
            int notUse = calculate(node.left) + calculate(node.right);
            int max = Math.max(notUse, use);
            memo.put(node, max);
            return max;

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