小知识，大挑战！本文正在参与“程序员必备小知识”创作活动。
描述
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 1:
Input: root = [3,9,20,null,null,15,7]
Output: 2
Example 2:
Input: root = [2,null,3,null,4,null,5,null,6]
Output: 5
Note:
The number of nodes in the tree is in the range [0, 10^5].
-1000 <= Node.val <= 1000
解析
根据题意，就是给出了一棵二叉树，要找到它的最小的深度，题目中给出了最小深度的定义，就是从根节点到最近的叶节点沿最短路径的节点个数。还是按照常规套路使用递归的方法进行求解。
思路比较简单：
- 如果 root 为空则直接返回 0
- 如果 root.left 为空则直接对 root.right 子树进行最小深度的查找
- 同理，如果root.right 为空则直接对 root.left 子树进行最小深度的查找
- 如果左右子树都不为空则对两个子树的深度求较小值即可。
解答
class TreeNode(object):
def __init__(self, val=0, left=None, right=None):
self.val = val
self.left = left
self.right = right
class Solution(object):
def minDepth(self, root):
"""
:type root: TreeNode
:rtype: int
"""
if not root: return 0
if not root.left:
return self.minDepth(root.right) + 1
if not root.right:
return self.minDepth(root.left) + 1
l = self.minDepth(root.left) + 1
r = self.minDepth(root.right) + 1
return min(l, r)
运行结果
Runtime: 1167 ms, faster than 19.36% of Python online submissions for Minimum Depth of Binary Tree.
Memory Usage: 95 MB, less than 22.36% of Python online submissions for Minimum Depth of Binary Tree.
解析
还可以使用栈的数据结构对节点进行迭代，找出最小的深度。
解答
class TreeNode(object):
def __init__(self, val=0, left=None, right=None):
self.val = val
self.left = left
self.right = right
class Solution(object):
def minDepth(self, root):
"""
:type root: TreeNode
:rtype: int
"""
if not root:
return 0
queue = collections.deque([(root, 1)])
while queue:
node, level = queue.popleft()
if node:
if not node.left and not node.right:
return level
else:
queue.append((node.left, level+1))
queue.append((node.right, level+1))
运行结果
Runtime: 692 ms, faster than 88.01% of Python online submissions for Minimum Depth of Binary Tree.
Memory Usage: 92.4 MB, less than 78.23% of Python online submissions for Minimum Depth of Binary Tree.
原题链接：leetcode.com/problems/mi…
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