【Leetcode-easy-543】Diameter of Binary Tree

这个是秒针系统的面试题，当时把题目理解错了，面试一个多小时最后挂了。
古有八股，今有Leetcode，加油。

题目

Given a binary tree, you need to compute the length of the diameter of the tree. The diameter of a binary tree is the length of the longest path between any two nodes in a tree. This path may or may not pass through the root.

Example:
Given a binary tree

          1         / \        2   3       / \           4   5    

Return 3, which is the length of the path [4,2,1,3] or [5,2,1,3].

Note: The length of path between two nodes is represented by the number of edges between them.

思路

对于每一个结点，经过它的最长路径的长度 = 它的左子树的最大深度 + 右子树的最大深度。

程序

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