统计树的高度 | 平衡二叉树的判断

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 depth of the two subtrees of every node never differ by 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
Return false.

思路

从下向上递归判断高度。

class Solution(object):
    def isBalanced(self, root):
        """
        :type root: TreeNode
        :rtype: bool
        """
        self.flag = True
        self.height(root)
        return self.flag
    
    def height(self, root):
        
        if root == None:
            return 0
        else:
            l_h = self.height(root.left) + 1
            r_h = self.height(root.right) + 1
            if abs(l_h - r_h) > 1:
                self.flag = False
            return max(l_h,r_h)