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.
Note: A leaf is a node with no children.
Example:
Given binary tree [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
return its minimum depth = 2.
这个题可以用BFS，也可以用DFS，关于具体思路可以见我的博客园的博客（C++版）leetcode111 C++
python代码：
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution:
def minDepth(self, root: TreeNode) -> int:
minnum = 1
if not root:
return 0
num = []
num.append(root)
while len(num):
length = len(num)
for i in range(length):
T = num[0]
num.pop(0)
if not T.left and not T.right:
return minnum
if T.left:
num.append(T.left)
if T.right: #不能写成elif!!!
num.append(T.right)
minnum += 1
return minnum
对了，切记，在if T.right上，不能写成elif T.right。试想，如果一个根有左右子结点，那么，用这个只能添加左子树，而右子树就被忽略了。
