111. Minimum Depth of Binary Tree

Edited by Charles

Easy · 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:

Reference to 102. Binary Tree Level Order Traversal Pay attention to the terminal condition, which is the current node has no either left node or right node. The counter for the min depth needs to be calculated before the iteration through the nodes in the same level.

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

Iteration:

1. Reference to 102. Binary Tree Level Order Traversal
2. Pay attention to the terminal condition, which is the current node has no either left node or right node. The counter for the min depth needs to be calculated before the iteration through the nodes in the same level.

# 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 minDepth(self, root: Optional[TreeNode]) -> int:
        if not root:
            return 0

        dq = deque([root])
        result = 0

        while dq:
            n = len(dq)
            result += 1

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

                if not node.left and not node.right:
                    return result

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

Recursion:

1. similar to 104. Maximum Depth of Binary Tree
2. pay attention to extreme cases: one side child node is None. The minimum depth is defined as the depth of the non-None side.

Study record

Field	Value
Attempts	1
Mastery	❚
The Latest Attempt	January 13, 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 minDepth(self, root: Optional[TreeNode]) -> int:
        return self.traverse(root)

    def traverse(self, node):
        if not node:
            return 0

        leftHeight = self.traverse(node.left)
        rightHeight = self.traverse(node.right)

        if node.left and not node.right:
            return 1 + leftHeight
        if not node.left and node.right:
            return 1 + rightHeight

        return 1 + min(leftHeight, rightHeight)

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