leetcode104 Maximum Depth of Binary Tree

Posted 2021-03-12 yawenw

 1 """
 2 Given a binary tree, find its maximum depth.
 3 The maximum depth is the number of nodes along the longest path from the root node down to the farthest 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 depth = 3.
13 """
14 """
15 此题好题，将提供三种解法
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(object):
24     def maxDepth(self, root):
25         if root == None:
26             return 0
27         else:
28             leftdepth = self.maxDepth(root.left)
29             rightdepth = self.maxDepth(root.right)
30             return max(leftdepth, rightdepth)+1
31 """
32 递归的方法，分别递归左子树，右子树的深度，取最大的+1(根结点)
33 """
34 class Solution2(object):
35     def maxDepth(self, root):
36         if root == None:
37             return 0
38         queue = []
39         depth = 0
40         queue.append(root)
41         while queue:
42             cur = [x.val if x else None for x in queue]
43             newqueue = []
44             for x in queue:
45                 if x.left:
46                     newqueue.append(x.left)  #bug 不是queue，应该是newqueue
47                 if x.right:
48                     newqueue.append(x.right)
49             queue = newqueue
50             depth += 1
51         return depth
52 
53 """用队列进行BFS搜索，记录depth"""
54 
55 class Solution3(object):
56     def maxDepth(self, root):
57         if root == None:
58             return 0
59         stack = []
60         depth = 0
61         stack.append((1, root))       #!!!用一个tuple存当前结点与其深度
62         while stack:
63             current_depth, root = stack.pop()     #cur 记录当前结点深度
64             if root:
65                 stack.append((current_depth+1, root.left))  #bug (,) 少加了一个括号
66                 stack.append((current_depth+1, root.right))
67                 depth = max(depth, current_depth)
68         return depth
69 
70 """
71 用栈进行DFS搜索，
72 值得注意的是：用(current_depth, root)入栈
73 记录当前结点的深度,每入栈一个结点 cur+1
74 """