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 longestpath 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： 典型的post-order traversal tree. For each node, we need to count the max edges of left tree and right tree. Then , plus them and compare with the max edges.

易错：在null return时候要 return -1. 还有只能return 单边最多的。

Time: O(n) n is nodes. Space: O(lg n)

/** * Definition for a binary tree node. * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public int diameterOfBinaryTree(TreeNode root) {        if(root == null){            return 0;        }        int[] max = new int[1];        countHelper(root, max);        return max[0];    }        private int countHelper(TreeNode root, int[] max){        if(root == null){            return -1;        }        int leftMax = countHelper(root.left, max) + 1;        int rightMax = countHelper(root.right, max) + 1;        int edges = leftMax + rightMax;        max[0] = Math.max(edges, max[0]);        return Math.max(leftMax, rightMax);    }}