House Robber 3 question

C++/House-Robber-3.cpp

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+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+
+// Question link: https://leetcode.com/problems/house-robber-iii/
+class Solution
+{
+public:
+    /*
+    Why is this DP on tree? And not simply greedy?
+    Try to think of some cases, like 5 -> 4 -> 2 -> 3
+    here greedy takes only alternate levels sum -> max(5 + 2, 4 + 3)
+     and misses the final answer as 5 + 3 (choosing the main root and last leaf) giving 8
+
+    Case 1: you choose the current node
+    Case 2: you dont choose the current node
+
+    Caching using map(unordered because its quicker than std::map & since we dont need nodes sorted in this question), to avoid TLE
+    */
+    unordered_map<TreeNode *, int> mp; // maximum value obtained for this node, stored as pairs in map
+    int helper(TreeNode *root)
+    {
+        if (root == NULL)
+            return 0;
+        if (mp.find(root) != mp.end())
+            return mp[root];
+        int case1 = root->val;
+        if (root->left)
+        {
+            case1 += helper(root->left->left);
+            case1 += helper(root->left->right);
+        }
+        if (root->right)
+        {
+            case1 += helper(root->right->right);
+            case1 += helper(root->right->left);
+        }
+        int case2 = helper(root->left) + helper(root->right);
+        return mp[root] = max(case1, case2);
+    }
+    int rob(TreeNode *root)
+    {
+        if (root == NULL)
+            return 0;
+        int ans = helper(root);
+        return ans;
+    }
+};