Delete leaves with a given value [DFS]

Time: O(N); Space: O(H); medium

Given a binary tree root and an integer target, delete all the leaf nodes with value target.

Note that once you delete a leaf node with value target, if it’s parent node becomes a leaf node and has the value target, it should also be deleted (you need to continue doing that until you can’t).

Example 1:

Input: root = {TreeNode} [1,2,3,2,null,2,4], target = 2

Output: {TreeNode} [1,null,3,null,4]

Explanation:

• Leaf nodes in green with value (target = 2) are removed (Picture in left).

• After removing, new nodes become leaf nodes with value (target = 2) (Picture in center).

Example 2:

Input: root = {TreeNode} [1,3,3,3,2], target = 3

Output: {TreeNode} [1,3,null,null,2]

Example 3:

Input: root = {TreeNode} [1,2,null,2,null,2], target = 2

Output: {TreeNode} [1]

Explanation:

• Leaf nodes in green with value (target = 2) are removed at each step.

Example 4:

Input: root = {TreeNode} [1,1,1], target = 1

Output: {TreeNode} []

Example 5:

Input: root = {TreeNode} [1,2,3], target = 1

Output: {TreeNode} [1,2,3]

Constraints:

• 1 <= target <= 1000

• Each tree has at most 3000 nodes.

• Each node’s value is between [1, 1000].

Hints:

1. Use the DFS to reconstruct the tree such that no leaf node is equal to the target. If the leaf node is equal to the target, return an empty object instead.

class TreeNode(object):
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

Auxiliary Tools

from graphviz import Graph

class TreeTasks(object):
    def visualize_tree(self, tree):
        def add_nodes_edges(tree, dot=None):
            # Create Graph (not Digraph) object
            if dot is None:
                dot = Graph()
                dot.node(name=str(tree), label=str(tree.val))
            # Add nodes
            if tree.left:
                dot.node(name=str(tree.left), label="."+str(tree.left.val))
                dot.edge(str(tree), str(tree.left))
                dot = add_nodes_edges(tree.left, dot=dot)
            if tree.right:
                dot.node(name=str(tree.right), label=str(tree.right.val)+".")
                dot.edge(str(tree), str(tree.right))
                dot = add_nodes_edges(tree.right, dot=dot)
            return dot
        # Add nodes recursively and create a list of edges
        dot = add_nodes_edges(tree)
        # Visualize the graph
        display(dot)
        return dot

class Solution1(object):
    """
    Time: O(N)
    Space: O(H)
    """
    def removeLeafNodes(self, root, target):
        """
        :type root: TreeNode
        :type target: int
        :rtype: TreeNode
        """
        if not root:
            return None

        root.left = self.removeLeafNodes(root.left, target)
        root.right = self.removeLeafNodes(root.right, target)

        return None if root.left == root.right and root.val == target else root

s = Solution1()

root = TreeNode(1)
root.left, root.right = TreeNode(2), TreeNode(3)
root.left.left = TreeNode(2)
root.right.left, root.right.right = TreeNode(2), TreeNode(4)
target = 2
tree = s.removeLeafNodes(root, target)
t = TreeTasks()
dot = t.visualize_tree(tree)

root = TreeNode(1)
root.left, root.right = TreeNode(3), TreeNode(3)
root.left.left, root.left.right = TreeNode(3), TreeNode(2)
target = 3
tree = s.removeLeafNodes(root, target)
t = TreeTasks()
dot = t.visualize_tree(tree)

root = TreeNode(1)
root.left = TreeNode(2)
root.left.left = TreeNode(2)
root.left.left.left = TreeNode(2)
target = 2
tree = s.removeLeafNodes(root, target)
t = TreeTasks()
dot = t.visualize_tree(tree)

root = TreeNode(1)
root.left, root.right = TreeNode(1), TreeNode(1)
target = 1
tree = s.removeLeafNodes(root, target)
assert tree == None

root = TreeNode(1)
root.left, root.right = TreeNode(2), TreeNode(3)
target = 1
tree = s.removeLeafNodes(root, target)
assert tree.val == 1
assert tree.left.val == 2
assert tree.right.val == 3

See also:

https://leetcode.com/problems/delete-leaves-with-a-given-value