思路:

• DFS枚举每个最高点(根节点)的左子树加右子树的深度和
• ans = Math.max(ans,left+right)

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