104. Maximum Depth of Binary Tree

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.

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计算二叉树的深度

java（3ms）：bfs

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public int maxDepth(TreeNode root) {
12         if (root == null)
13             return 0 ;
14         int depth = 0 ;
15         TreeNode node = null ;
16         Queue<TreeNode> que = new LinkedList<TreeNode>() ;
17         que.offer(root) ;
18         while(!que.isEmpty()){
19             int size = que.size() ;
20             for (int i = 0 ; i < size ; i++){
21                 node = que.poll() ;
22                 if (node.left != null){
23                     que.offer(node.left) ;
24                 }
25                 if (node.right != null){
26                     que.offer(node.right) ;
27                 }
28                 
29             }
30             depth++ ;
31         }
32         return depth;
33     }
34 }

C++（6ms）：

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     int maxDepth(TreeNode* root) {
13         if(root == NULL)
14           return 0 ;
15         queue<TreeNode*> q;
16         q.push(root) ;
17         int dep = 0 ;
18         while(!q.empty()){
19            int len = q.size();//很关键，队列pop后size会动态变化
20            for(int i = 0 ; i < len;i++){
21                TreeNode* t =  q.front();
22                q.pop();
23                if(t->left != NULL)
24                   q.push(t->left) ;
25                if(t->right != NULL)
26                   q.push(t->right) ;
27            }
28            dep++ ;
29         }
30         return dep ;
31     }
32 };

java（1ms）：dfs

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public int maxDepth(TreeNode root) {
12         if (root == null)
13             return 0 ;
14         
15         return Math.max(maxDepth(root.left),maxDepth(root.right)) + 1 ;
16     }
17 }

C++(6ms)dfs:

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     int maxDepth(TreeNode* root) {
13         if (root == NULL)
14           return 0 ;
15         return max(maxDepth(root->left),maxDepth(root->right)) +1 ;
16     }
17 };