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

Given binary tree A = {3,9,20,#,#,15,7}, B = {3,#,20,15,7}

A)  3            B)    3
   / \                  \
  9  20                 20
    /  \                / \
   15   7              15  7

The binary tree A is a height-balanced binary tree, but B is not.

 /**
  * Definition of TreeNode:
  * public class TreeNode {
  *     public int val;
  *     public TreeNode left, right;
  *     public TreeNode(int val) {
  *         this.val = val;
  *         this.left = this.right = null;
  *     }
  * }
  */
 public class Solution {
     /**
      * @param root: The root of binary tree.
      * @return: True if this Binary tree is Balanced, or false.
      */
     public boolean isBalanced(TreeNode root) {
         if (root == null) return true;
         if (Math.abs(height(root.left) - height(root.right)) > ) return false;
         return isBalanced(root.left) && isBalanced(root.right);
     }  

     public int height(TreeNode root) {
         if (root == null) return ;
         return Math.max(height(root.left), height(root.right)) + ;
     }
 }

 public class Solution {
     public boolean isBalanced(TreeNode root) {
         if (root == null) return true;
         return helper(root) != -;
     }

     public int helper(TreeNode root) {
         if (root == null) return ;
         int leftHeight = helper(root.left);
         int rightHeight = helper(root.right);

         if (leftHeight == - || rightHeight == -) return -;
         if (Math.abs(leftHeight - rightHeight) > ) return -;

         return Math.max(helper(root.left), helper(root.right)) + ;
     }
 }