给定一棵二叉树，你需要计算它的直径长度。一棵二叉树的直径长度是任意两个结点路径长度中的最大值。这条路径可能穿过也可能不穿过根结点。

答案：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    int max = 0;
    public int diameterOfBinaryTree(TreeNode root) {
        dfs(root);
        return max;
    }
    public int dfs(TreeNode root) {
        if(root.left == null && root.right == null) {
            return 0;
        }
        int leftSize = root.left == null ? 0 : dfs(root.left) + 1;
        int rightSize = root.right == null ? 0 :dfs(root.right) + 1;
        max = Math.max(max, leftSize + rightSize);
        return Math.max(leftSize, rightSize);
    }
}