513. Find Bottom Left Tree Value

Edited by Charles

Medium · Python · Binary Tree

Problem

Read the original problem on LeetCode.

The official page contains the problem statement, examples, and constraints.

Explanation, examples & other approaches

Core idea

Iteration:

The most fabulous point of this solution is, when we iterate all nodes in the same level, the first valid/non-None node is the leftmost node so far.

Original study notes follow, including intermediate snippets and earlier attempts.

Iteration:

The most fabulous point of this solution is, when we iterate all nodes in the same level, the first valid/non-None node is the leftmost node so far.

# 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
from collections import deque

class Solution:
    def findBottomLeftValue(self, root: Optional[TreeNode]) -> int:
        dq = deque([root])
        result = 0

        while dq:
            n = len(dq)

            for i in range(n):
                node = dq.popleft()

                if i == 0:
                    result = node.val

                if node.left:
                    dq.append(node.left)
                if node.right:
                    dq.append(node.right)

        return result

Recursion

The tricky point in this solution is:

1. Once we meet a node without any left and right children, we update maxDepth and result.
2. Once we traverse all nodes, the maxDepth and result are definitely the final outcome.
3. We do not need to worry that the maxDepth and result are overwritten by the right node with the same parent node, since maxDepth has been updated already and the right node has the same depth so it won’t trigger the update operation.

# 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 findBottomLeftValue(self, root: Optional[TreeNode]) -> int:
        self.result = root.val
        self.maxDepth = 0

        self.traverse(root, 0)

        return self.result

    def traverse(self, node, depth):
        if not node.left and not node.right:
            if depth > self.maxDepth:
                self.maxDepth = depth
                self.result = node.val
            return

        if node.left:
            self.traverse(node.left, depth + 1)
        if node.right:
            self.traverse(node.right, depth + 1)

BFS 解法

题解链接 https://leetcode.cn/problems/find-bottom-left-tree-value/solutions/2049776/bfs-wei-shi-yao-yao-yong-dui-lie-yi-ge-s-f34y/

Study record

Field	Value
Attempts	1
Mastery	❚
The Latest Attempt	January 15, 2026

Python 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 findBottomLeftValue(self, root: Optional[TreeNode]) -> int:
        q = deque([root])

        while q:
            node = q.popleft()
            # 先右后左
            if node.right: q.append(node.right)
            if node.left: q.append(node.left)

        return node.val

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