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; }
 * }
 */
public class Solution {
    int ans = 0;
    public int diameterOfBinaryTree(TreeNode root) {
        if (root == null) return 0;
        
        helper(root);
        return ans;
    }
    private int helper(TreeNode root) {
        if (root == null) return 0;
        if (root.left == null && root.right == null) return 1;
        int left = helper(root.left);
        int right = helper(root.right);
        //Math.max函数保证不断在递归里进行更新ans的最大值
        ans = Math.max(ans, left + right);
        return Math.max(left, right) + 1;
    }
}