0110. Balanced Binary Tree
https://leetcode.com/problems/balanced-binary-tree
Description
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:

**Input:** root = [3,9,20,null,null,15,7]
**Output:** true
Example 2:

**Input:** root = [1,2,2,3,3,null,null,4,4]
**Output:** false
Example 3:
**Input:** root = []
**Output:** true
Constraints:
The number of nodes in the tree is in the range
[0, 5000]
.-104 <= Node.val <= 104
ac
O(n), traversal is always O(n)
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public boolean isBalanced(TreeNode root) {
if (root == null) return true;
return height(root) != -1;
}
private int height(TreeNode node) {
if (node == null) return 0;
int left = height(node.left);
int right = height(node.right);
if (left == -1 || right == -1 || Math.abs(left - right) > 1) return -1;
return left > right ? 1 + left : 1 + right;
}
}
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