111、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.
解题思路：

# 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
"""
def dfs(root,length):
if(root==None):return
length += 1
if(root.left==None and root.right==None):
res.append(length)
return
dfs(root.left,length)
dfs(root.right,length)
res = []
dfs(root,0)
if(res==[]):return 0
return min(res)