110. Balanced Binary Tree

Given a binary tree, determine if it is height-balanced.

For this problem, a height-balanced binary tree is defined as:

a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

Example 1:

Given the following tree [3,9,20,null,null,15,7]:

    3
   / \
  9  20
    /  \
   15   7

Return true. Example 2:

Given the following tree [1,2,2,3,3,null,null,4,4]:

       1
      / \
     2   2
    / \
   3   3
  / \
 4   4

Return false.

// Top-Down Recursion
int helper(TreeNode* node) {
    if (!node) return 0;
    return max(helper(node->left), helper(node->right)) + 1;
}
bool isBalanced(TreeNode* root) { // time: O(n^2); space: O(n)
    if (!root) return true;
    int left = helper(root->left);
    int right = helper(root->right);
    return abs(left - right) <= 1 && isBalanced(root->left) && isBalanced(root->right);
}

// Bottom-Up Recursion
int helper(TreeNode* node) {
    if (!node) return 0;
    int left = helper(node->left);
    if (left == -1) return -1;
    int right = helper(node->right);
    if (right == -1) return -1;
    if (abs(left - right) > 1) return -1;
    return max(left, right) + 1;
}
bool isBalanced(TreeNode* root) { // time: O(n); space: O(n)
    return helper(root) != -1;
}