LeetCode - Diameter of Binary Tree

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.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
/*
二叉树的直径：二叉树中从一个结点到另一个节点最长的路径，叫做二叉树的直径
采用分治和递归的思想：根节点为root的二叉树的直径 = Max(左子树直径，右子树直径，左子树的最大深度（不包括根节点）+右子树的最大深度（不包括根节点）+1)
*/

class Solution {
    int res ;
    public int diameterOfBinaryTree(TreeNode root) {
        res = 0;
        getDepth(root);
        return res;
    }
    
	// 此函数是返回树的最大深度
    public int getDepth(TreeNode root){
        if(root == null){
            return 0;
        }
        int left = getDepth(root.left);
        int right = getDepth(root.right);
        res = Math.max(res, left+right);
        return Math.max(left, right) + 1;
        
    } 
}