Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

 1 struct TreeNode {
 2     int            val;
 3     TreeNode*    left;
 4     TreeNode*    right;
 5     TreeNode(int x): val(x), left(NULL),right(NULL) {}
 6 };
 7 
 8 //采用DFS的思想
 9 int maxDepth(TreeNode *root)
10 {
11     if (NULL == root)
12         return 0;
13     int l = maxDepth(root->left);
14     int r = maxDepth(root->right);
15 
16     return l > r ? l + 1:r+1;
17     //以上这两种方式有一种更简便的方法
18     //return 1 + max(maxDepth(root->left), maxDepth(root->right));
19 }
20 
21 //采用BFS的方法，引入队列
22 int maxDepth(TreeNode *root)
23 {
24     if (NULL == root)
25         return 0;
26     queue <TreeNode *> que;
27     int nCount = 1;
28     int nDepth = 0;// 记录队列里面每一层上的元素
29 
30     que.push(root);
31     while(!que.empty()) {
32         TreeNode *pTemp = que.front();
33         que.pop();
34         nCount --;
35 
36         if (pTemp->left)
37             que.push(pTemp->left);
38         if (pTemp->right)
39             que.push(pTemp->right);
40         
41         if (nCount == 0) {
42             nDepth ++;
43             nCount = que.size();
44         }
45     }
46     return nDepth;
47 }