```
# Definition for a binary tree node.
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution(object):
def minDepth(self, root):
"""
:type root: TreeNode
:rtype: int
"""
if not root:
return 0
queue = [(root, 1)]
length = len(queue)
ans = []
while length:
# put question specific code here for BFS problems
# get the average of the current level
# enqueue the children of the all nodes
# in the current level
tmpq = []
for t in queue:
node = t[0]
if not (node.left or node.right):
return t[1]
if node.left:
tmpq.append((node.left, t[1]+1))
if node.right:
tmpq.append((node.right, t[1]+1))
# reassign the queue
queue = tmpq
length = len(queue)
```