1181 · Diameter of Binary Tree

求二叉树任意两节点之间距离的最大值
自己写的，这时间复杂度不得上天了，结果竟然击败了81%

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */

class Solution {
public:
    /**
     * @param root: a root of binary tree
     * @return: return a integer
     */
    int diameterOfBinaryTree(TreeNode * root) {
        // write your code here
        if(root == nullptr)
            return 0;
        int a = helper(root->left) + helper(root->right);
        int b = max(diameterOfBinaryTree(root->left), diameterOfBinaryTree(root->right));
        return max(a,b);
    }

    int helper(TreeNode *root) {
        if(root == nullptr)
            return 0;
        return max(helper(root->left) + 1, helper(root->right) + 1);
    }
};