LeetCode 1110: Delete Nodes And Return Forest Solution

Master LeetCode problem 1110 (Delete Nodes And Return Forest), a medium challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.

1110. Delete Nodes And Return Forest

Medium

Array Hash Table Tree Depth-First Search Binary Tree

Problem Explanation

Explanation

To solve this problem, we can perform a depth-first search (DFS) traversal of the binary tree. At each node, we check if the node's value is in the to_delete list. If it is, we mark the node for deletion and recursively process its left and right child nodes. If the current node itself is not in the to_delete list but its parent is, we add the current node to the result as a new tree root. We continue this process until we have traversed all nodes of the tree.

Algorithm:

Initialize an empty list to store the result forest.
Perform a recursive DFS traversal of the tree.
At each node:
- Check if the node's value is in the to_delete list.
- If it is, mark the node for deletion and recursively process its left and right child nodes.
- If the node is not marked for deletion but its parent is, add the node to the result as a new tree root.
Return the list of root nodes of the remaining trees in the forest.

Time Complexity:

The time complexity of this approach is O(N), where N is the number of nodes in the binary tree.

Space Complexity:

The space complexity is also O(N) due to the recursive calls on the call stack and the space required to store the result forest.

Solution Code

class Solution {
    public List<TreeNode> delNodes(TreeNode root, int[] to_delete) {
        List<TreeNode> result = new ArrayList<>();
        Set<Integer> toDeleteSet = new HashSet<>();
        for (int val : to_delete) {
            toDeleteSet.add(val);
        }
        dfs(root, toDeleteSet, result, true);
        return result;
    }
    
    private TreeNode dfs(TreeNode node, Set<Integer> toDeleteSet, List<TreeNode> result, boolean isRoot) {
        if (node == null) return null;
        
        boolean deleted = toDeleteSet.contains(node.val);
        if (isRoot && !deleted) {
            result.add(node);
        }
        
        node.left = dfs(node.left, toDeleteSet, result, deleted);
        node.right = dfs(node.right, toDeleteSet, result, deleted);
        
        return deleted ? null : node;
    }
}

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