Leetcode 1218 Solution

This article provides solution to leetcode question 1218 (lowest-common-ancestor-of-deepest-leaves)
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https://leetcode.com/problems/lowest-common-ancestor-of-deepest-leaves

Solution

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def lcaDeepestLeaves(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
        max_depth = 0
        max_depth_nodes = set()



        def dfs(node, depth=0):
            nonlocal max_depth
            nonlocal max_depth_nodes



            if not node:
                return



            if depth > max_depth:
                max_depth = depth
                max_depth_nodes = {node}
            elif depth == max_depth:
                max_depth_nodes.add(node)



            dfs(node.left, depth + 1)
            dfs(node.right, depth + 1)



        dfs(root)



        ans = None
        def find(node):
            nonlocal max_depth_nodes
            nonlocal ans



            if not node:
                return 0



            left_cnt = find(node.left)
            right_cnt = find(node.right)



            if left_cnt > 0 and right_cnt > 0 and left_cnt + right_cnt == len(max_depth_nodes):
                ans = node
            elif left_cnt == 0 and right_cnt == 0 and node in max_depth_nodes and len(max_depth_nodes) == 1:
                ans = node



            return int(node in max_depth_nodes) + left_cnt + right_cnt



        find(root)



        return ans