LeetCode开心刷题五十二天——110. Balanced Binary Tree

110. Balanced Binary Tree Easy

Given a binary tree, determine if it is height-balanced.

For this problem, a height-balanced binary tree is defined as:

a binary tree in which the left and right subtrees of every node differ in height by no more than 1.

 

Example 1:

Given the following tree [3,9,20,null,null,15,7]:

    3
   / \
  9  20
    /  \
   15   7

Return true.

Example 2:

Given the following tree [1,2,2,3,3,null,null,4,4]:

       1
      / \
     2   2
    / \
   3   3
  / \
 4   4
这个题很明显是用DFS做,问题在于深度信息怎么传递,本来我想着在TreeNode中加一个变量信息
但这样明显有违题意,所以代码中采用了新的函数return直接返回深度或者-1,返回-1并不代表深度,只是
表示程序结束了。此外还是设置了一个类内全局变量来表示,这个题的延伸或者说目前的简单之处就在于
他只问你是否平衡而不需要问具体深度,所以只要用一个变量表示平衡,然后判断当数值不平衡时,改变变量即可
class TreeNode(object):
    def __init__(self,x):
        self.val=x
        self.left=None
        self.right=None
        
class Solution(object):
    def isBalanced(object,root):
        self.balanced=True
        def height(root):
            if not root or not self.balanced:
                return -1
            l=height(root.left)
            r=height(root.right)
            if abs(l-r)>1:
                self.balanced=False
                return -1
            return max(l,r)+1
        height(root)
        return self.balanced

 

 

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