543. 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.

递归解题。遍历整个数，根据题意，直径等于左子树深度加上右子树深度，实时更新max，返回值是左右子树较大的深度值加1。代码如下：

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