[LeetCode] Maximum Depth of Binary Tree

Well, this problem has the highest acceptance rate among all OJ problems. It has a very easy 1-line reursive solution. I am not sure whether this one can be called "DFS" so I only call it "recursive".

1 class Solution { 
2 public:
3     int maxDepth(TreeNode* root) {
4         return root ? 1 + max(maxDepth(root -> left), maxDepth(root -> right)) : 0;
5     }
6 };

Well, you may also solve it using a level-order traversal (BFS) with a queue.

 1 class Solution {
 2 public:
 3     int maxDepth(TreeNode* root) {
 4         int depth = 0;
 5         if (!root) return depth;
 6         queue<TreeNode*> level;
 7         level.push(root);
 8         while (!level.empty()) {
 9             depth++;
10             int n = level.size();
11             for (int i = 0; i < n; i++) {
12                 TreeNode* node = level.front();
13                 level.pop();
14                 if (node -> left) level.push(node -> left);
15                 if (node -> right) level.push(node -> right);
16             }
17         }
18         return depth; 
19     } 
20 };