is_balanced_bottom_up.cpp

//T(n) = O(n^2)
int maxDepth(TreeNode *root) {
    if (root == NULL) {
        return 0;
    }
    int leftDepth = 0;
    int rightDepth = 0;
    leftDepth = maxDepth(root->left);
    rightDepth = maxDepth(root->right);
    return 1 + max(leftDepth, rightDepth);
}


public boolean isBalanced(TreeNode* root) {
    if (root == NULL) { 
        return true;
    }
    return abs(maxDepth(root->left) - maxDepth(root->right)) <= 1 && isBalanced(root->left) && isBalanced(root->right);
}