题目

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.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

  1.     3
  2.    / \
  3.   9  20
  4.     /  \
  5.    15   7

return its depth = 3.

题意

求二叉树的最大深度。

思路

DFS或者BFS。

代码实现

Java

DFS

  1. class Solution {
  2.     public int maxDepth(TreeNode root) {
  3.         return root == null ? 0 : Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
  4.     }
  5. }

BFS

  1. class Solution {
  2.     public int maxDepth(TreeNode root) {
  3.         Queue<TreeNode> q = new ArrayDeque<>();
  4.         int depth = 0;
  5.         if (root != null) {
  6.             q.offer(root);
  7.         }
  9.         while (!q.isEmpty()) {
  10.             depth++;
  11.             int size = q.size();
  12.             for (int i = 0; i < size; i++) {
  13.                 TreeNode cur = q.poll();
  14.                 if (cur.left != null) q.offer(cur.left);
  15.                 if (cur.right!=null) q.offer(cur.right);
  16.             }
  17.         }
  19.         return depth;
  20.     }
  21. }

JavaScript

  1. /**
  2.  * @param {TreeNode} root
  3.  * @return {number}
  4.  */
  5. var maxDepth = function (root) {
  6.   return root ? Math.max(maxDepth(root.left), maxDepth(root.right)) + 1 : 0
  7. }