1 """
2 Given a binary tree, find its minimum depth.
3 The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.
4 Note: A leaf is a node with no children.
5 Example:
6 Given binary tree [3,9,20,null,null,15,7],
7 3
8 / \
9 9 20
10 / \
11 15 7
12 return its minimum depth = 2.
13 """
14 """
15 解法一：迭代，用一个(node,depth) 与树结构绑定 并持续更新
16 """
17 class TreeNode:
18 def __init__(self, x):
19 self.val = x
20 self.left = None
21 self.right = None
22
23 class Solution1:
24 def minDepth(self, root):
25 if not root:
26 return 0
27 queue = []
28 queue.append((root, 1)) #!!!关键记录深度
29 res = float('inf') #结果初始化为最大
30 while queue:
31 node, depth = queue.pop(0)
32 if not node.left and not node.right:
33 res = min(res, depth)
34 if node.left:
35 queue.append((node.left, depth+1))
36 if node.right:
37 queue.append((node.right, depth+1))
38 return res
39 """
40 解法二：递归
41 """
42 class Solution2:
43 def minDepth(self, root):
44 if not root:
45 return 0
46 if not root.left:
47 return self.minDepth(root.right)+1
48 if not root.right:
49 return self.minDepth(root.left)+1
50 return min(self.minDepth(root.left), self.minDepth(root.right))+1
