leetcode111 Minimum Depth of Binary Tree

 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