110. Balanced Binary Tree

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.

//Time: O(n!), Space: O(h)   
 public boolean isBalanced(TreeNode root) {
        if (root == null) {
            return true;
        }
        
        if (Math.abs(depth(root.left) - depth(root.right)) > 1) {
            return false;
        }
        
        return isBalanced(root.left) && isBalanced(root.right);
    }
    
    private int depth(TreeNode root) {
        if (root == null) {
            return 0;
        }
        
        return 1 + Math.max(depth(root.left), depth(root.right));
    }