Max Depth of Binary Tree

• Time : $O(n)$
• Space : $O(n)$

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
int helper(TreeNode* node, int now) {
    if(!node) return now;
    return max(helper(node->left, now + 1), helper(node->right, now + 1));
}
int Solution::maxDepth(TreeNode* A) {
    return A != NULL ? helper(A,0) : 0;
}