111. 二叉树的最小深度

BFS模板的应用。

总结的DFS、BFS模板参考：https://www.cnblogs.com/panweiwei/p/13065661.html

class Solution(object):
    def minDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        # 特判：根节点为空
        if not root:
            return 0
        # 特判：只有根节点
        if not root.left and not root.right:
            return 1
        res = 1
        # BFS
        stack = [root]
        while stack:
            lenstack = len(stack)
            for i in range(lenstack):
                node = stack.pop(0)
                if not node.left and not node.right:
                    return res
                if node.left:
                    stack.append(node.left)
                if node.right:
                    stack.append(node.right)
            res += 1
        return res