leetcode104 Maximum Depth of Binary Tree

"""
Given a binary tree, find its maximum depth.
The maximum depth is the number of nodes along the longest path from the root node down to the farthest 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 depth = 3.
"""
"""
此题好题，将提供三种解法
"""
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution1(object):
    def maxDepth(self, root):
        if root == None:
            return 0
        else:
            leftdepth = self.maxDepth(root.left)
            rightdepth = self.maxDepth(root.right)
            return max(leftdepth, rightdepth)+1
"""
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)
"""
class Solution2(object):
    def maxDepth(self, root):
        if root == None:
            return 0
        queue = []
        depth = 0
        queue.append(root)
        while queue:
            cur = [x.val if x else None for x in queue]
            newqueue = []
            for x in queue:
                if x.left:
                    newqueue.append(x.left)  #bug 不是queue，应该是newqueue
                if x.right:
                    newqueue.append(x.right)
            queue = newqueue
            depth += 1
        return depth

"""用队列进行BFS搜索，记录depth"""

class Solution3(object):
    def maxDepth(self, root):
        if root == None:
            return 0
        stack = []
        depth = 0
        stack.append((1, root))       #!!!用一个tuple存当前结点与其深度
        while stack:
            current_depth, root = stack.pop()     #cur 记录当前结点深度
            if root:
                stack.append((current_depth+1, root.left))  #bug (,) 少加了一个括号
                stack.append((current_depth+1, root.right))
                depth = max(depth, current_depth)
        return depth

"""
用栈进行DFS搜索，
值得注意的是：用(current_depth, root)入栈
记录当前结点的深度,每入栈一个结点 cur+1
"""