111. Minimum Depth of Binary Tree

原文题目：

111. Minimum Depth of Binary Tree

读题：

求最小深度，这和求树的最大深度差不多，只不过需要注意当左子树或者右子树为空时，最小深度并不为0，而是取决于非空子树的最小深度

# 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
        lengthL = self.minDepth(root.left)
        lengthR = self.minDepth(root.right)
        if not lengthL:
            return lengthR + 1
        if not lengthR:
            return lengthL + 1
        return min(lengthR,lengthL) + 1