Leet Code Daily 2024-07-17 - Delete Nodes and Return Forest

/**
 * 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) {}
 * };
 */

constexpr int kDeletedValue = -1;

class Solution {
public:
    void scan(TreeNode* root, unordered_map<int, TreeNode*>& parent_map,
              unordered_map<int, TreeNode*>& node_map) {
        if (root == nullptr) {
            return;
        }
        node_map[root->val] = root;
        if (root->left != nullptr) {
            parent_map[root->left->val] = root;
            scan(root->left, parent_map, node_map);
        }
        if (root->right != nullptr) {
            parent_map[root->right->val] = root;
            scan(root->right, parent_map, node_map);
        }
    }

    vector<TreeNode*> delNodes(TreeNode* root, vector<int>& to_delete) {

        unordered_map<int, TreeNode*> parent_map, node_map;
        scan(root, parent_map, node_map);

        for (const auto& val : to_delete) {
            node_map[val]->val = kDeletedValue;
            if (!parent_map.contains(val)) {
                continue;
            }
            if (parent_map[val]->left == node_map[val]) {
                parent_map[val]->left = nullptr;
            }
            if (parent_map[val]->right == node_map[val]) {
                parent_map[val]->right = nullptr;
            }
        }

        vector<TreeNode*> ret;
        if (root->val != kDeletedValue) {
            ret.push_back(root);
        }
        for (const auto& pair : parent_map) {
            if (node_map[pair.first]->val != kDeletedValue &&
                pair.second->val == kDeletedValue) {
                ret.push_back(node_map[pair.first]);
            }
        }
        return ret;
    }
};