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;

    }
}