Minimum Depth of Binary Tree

题目：

Given a binary tree, find its minimum depth.

The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.

思路，判断每层节点是否都有孩子，如果左右孩子都为空，则跳出循环，返回树的高度。

class Solution(object):
    def minDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        from collections import deque
        
        if not root:
            return 0
        
        queue = deque([root])
        flag = 'flag'
        queue.append(flag)
        
        mindepth = 1
        while queue:
            node = queue.popleft()
            
            if node == flag :
                mindepth += 1
                if not queue:
                    break
                queue.append(flag)
            else:
                if not node.left and not node.right:
                    return mindepth
                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)