Maximum Depth of Binary Tree（二叉树最大深度）

来源：https://leetcode.com/problems/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.

1. 深度遍历，递归

Java

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

Python

 # Definition for a binary tree node.
 # class TreeNode(object):
 #     def __init__(self, x):
 #         self.val = x
 #         self.left = None
 #         self.right = None

 class Solution(object):
     def maxDepth(self, root):
         """
         :type root: TreeNode
         :rtype: int
         """
         if not root:
             return 0
         left_depth = self.maxDepth(root.left) + 1
         right_depth = self.maxDepth(root.right) + 1
         return left_depth if left_depth>right_depth else right_depth

2. 广度遍历（逐层遍历），使用队列

Java

 /**
  * Definition for a binary tree node.
  * public class TreeNode {
  *     int val;
  *     TreeNode left;
  *     TreeNode right;
  *     TreeNode(int x) { val = x; }
  * }
  */
 class Solution {
     public int maxDepth(TreeNode root) {
         Queue<TreeNode> queue = new LinkedList<TreeNode>();
         int depth = -1;
         queue.offer(root);
         TreeNode node = null;
         int queueSize = 0;
         while(!queue.isEmpty()) {
             queueSize = queue.size();
             while(queueSize-- > 0) {
                 node = queue.poll();
                 if(node == null) {
                     continue;
                 }
                 queue.offer(node.left);
                 queue.offer(node.right);
             }
             depth += 1;
         }
         return depth;
     }
 }

Python

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def maxDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        queue = [root]
        depth = -1
        while queue:
            depth += 1
            queue = [kid for node in queue if node for kid in (node.left, node.right)]
        return depth