minimum_depth_of_binary_tree.py

# Definition for a binary tree node.
class TreeNode(object):
  def __init__(self, val=0, left=None, right=None):
    self.val = val
    self.left = left
    self.right = right

from collections import deque

class Solution(object):
  def minDepth(self, root):
    if not root:
      return 0
  
    queue = deque([(root, 1)])

    while queue:
      node, depth = queue.popleft()

      if not node.left and not node.right:
        return depth
      
      if node.left:
        queue.append((node.left, depth + 1))

      if node.right:
        queue.append((node.right, depth + 1))