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

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int maxDepth(TreeNode root) {
        if (root == null)
            return 0 ;
        int depth = 0 ;
        TreeNode node = null ;
        Queue<TreeNode> que = new LinkedList<TreeNode>() ;
        que.offer(root) ;
        while(!que.isEmpty()){
            int size = que.size() ;
            for (int i = 0 ; i < size ; i++){
                node = que.poll() ;
                if (node.left != null){
                    que.offer(node.left) ;
                }
                if (node.right != null){
                    que.offer(node.right) ;
                }
                
            }
            depth++ ;
        }
        return depth;
    }
}

C++（6ms）：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int maxDepth(TreeNode* root) {
        if(root == NULL)
          return 0 ;
        queue<TreeNode*> q;
        q.push(root) ;
        int dep = 0 ;
        while(!q.empty()){
           int len = q.size();//很关键，队列pop后size会动态变化
           for(int i = 0 ; i < len;i++){
               TreeNode* t =  q.front();
               q.pop();
               if(t->left != NULL)
                  q.push(t->left) ;
               if(t->right != NULL)
                  q.push(t->right) ;
           }
           dep++ ;
        }
        return dep ;
    }
};

java（1ms）：dfs

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int maxDepth(TreeNode root) {
        if (root == null)
            return 0 ;
        
        return Math.max(maxDepth(root.left),maxDepth(root.right)) + 1 ;
    }
}

 C++(6ms)dfs:

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int maxDepth(TreeNode* root) {
        if (root == NULL)
          return 0 ;
        return max(maxDepth(root->left),maxDepth(root->right)) +1 ;
    }
};