// Version 1: with ResultType
/**
 * 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;
 *     }
 * }
 */
class ResultType {
    public boolean isBalanced;
    public int maxDepth;
    public ResultType(boolean isBalanced, int maxDepth) {
        this.isBalanced = isBalanced;
        this.maxDepth = maxDepth;
    }
}

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) {
        return helper(root).isBalanced;
    }

    private ResultType helper(TreeNode root) {
        if (root == null) {
            return new ResultType(true, 0);
        }

        ResultType left = helper(root.left);
        ResultType right = helper(root.right);

        // subtree not balance
        if (!left.isBalanced || !right.isBalanced) {
            return new ResultType(false, -1);
        }

        // root not balance
        if (Math.abs(left.maxDepth - right.maxDepth) > 1) {
            return new ResultType(false, -1);
        }

        return new ResultType(true, Math.max(left.maxDepth, right.maxDepth) + 1);
    }
}

// Version 2: without ResultType
public class Solution {
    public boolean isBalanced(TreeNode root) {
        return maxDepth(root) != -1;
    }

    private int maxDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }

        int left = maxDepth(root.left);
        int right = maxDepth(root.right);
        if (left == -1 || right == -1 || Math.abs(left-right) > 1) {
            return -1;
        }
        return Math.max(left, right) + 1;
    }
}