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.

对于在一个树里找某种路径的问题，一般在dfs/递归的时候需要考虑两种情况，一种是可以继续用来拼成一个路径的结果，另一种是不可以继续用于拼成一个路径的结果。

/**
 * 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[] res = getDiameter(root);

        return Math.max(res[0], res[1]) - 1;
    }

    public int[] getDiameter(TreeNode root) {
        int[] res = new int[2];
        if(root == null) {
            res[0] = 0;
            res[1] = Integer.MIN_VALUE;
            return res;
        }

        int[] left = getDiameter(root.left);
        int[] right = getDiameter(root.right);

        res[0] = 1 + Math.max(left[0], right[0]);
        res[1] = Math.max(left[1], right[1]);
        res[1] = Math.max(res[1], 1 + left[0] + right[0]);

        return res;
    }
}