4.1 Implement a function to check if a binary tree is balanced. For the purposes of this question, a balanced tree is defined to be a tree such that the heights of the two subtrees of any node never differ by more than one.

4.1 isBalanced

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

private int helper(TreeNode root) {
    if (root == null) {
        return 0;
    }
    
    int left = helper(root.left);
    if (left == -1) {
        return -1;
    }
    
    int right = helper(root.right);
    if (right == -1) {
        return -1;
    }
    
    if (Math.abs(left - right) > 1) {
        return -1;
    }
    else {
        return Math.max(left, right) + 1;
    }
}