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:

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

class Solution {
boolean isBalanced = true;
public boolean isBalanced(TreeNode root) {
helper(root);
return isBalanced;
}
private int helper(TreeNode node) {
if (node == null) return 0;
int left = helper(node.left);
int right = helper(node.right);
if (Math.abs(left - right) > 1) {
isBalanced = false;
}
return Math.max(left, right) + 1;
}
}

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