1110. Delete Nodes And Return Forest

Hint - Recursion

/**
 * 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) {}
 * };
 */
class Solution {
public:
    vector<TreeNode*> delNodes(TreeNode* root, vector<int>& to_delete)
    {
        // Vector to store the resulting forest of trees
        // Convert the list of nodes to delete into a set for faster lookup
        // Call the helper function to process the tree
        // If the root is not deleted, add it to the forest
        // Return the forest
    }

    // Helper function to process each node
    helper()
    {
        // If the node is null, return null

        // Recursively process the left and right subtrees

        // If the current node is not in the set of nodes to delete, return it
        
        // If the current node is to be deleted, add its children to the forest if they exist

        // Delete the current node
        // Return null to indicate the node has been deleted
    }    
};

Solution - Recursion

/**
 * 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) {}
 * };
 */
class Solution {
public:
    vector<TreeNode*> delNodes(TreeNode* root, vector<int>& to_delete)
    {
        vector<TreeNode*> forest;
        unordered_set<int> st(to_delete.begin(), to_delete.end());
        root = helper(root, st, forest);
        if (root) forest.push_back(root);
        return forest;        
    }

    TreeNode* helper(TreeNode* node, unordered_set<int>& st, vector<TreeNode*>& forest)
    {
        if (!node) return nullptr;

        node->left = helper(node->left, st, forest);
        node->right = helper(node->right, st, forest);

        if (!st.count(node->val)) return node;
        
        if (node->left) forest.push_back(node->left);
        if (node->right) forest.push_back(node->right);
        delete node;
        return nullptr;
    }    
};

• 時間複雜度：
  $O(n)$
• 空間複雜度：
  $O(n)$