0110. Balanced Binary Tree

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public int helper(TreeNode root) {
        if (root == null) {
            return 0;
        }
        int left = helper(root.left);
        int right = helper(root.right);
        if (left == -1 || right == -1) {
            return -1;
        }
        int diff = Math.abs(left - right);
        if (diff > 1) {
            return -1;
        }
        else {
            return Math.max(left, right) + 1;
        }
    }

    public boolean isBalanced(TreeNode root) {
        if (root == null) {
            return true;
        }
        return helper(root) == -1 ? false : true;
    }
}