没有解决，参考solution解决。本质仍然是Binary Tree 的遍历问题

first.cpp

//效率不算高
//Runtime: 8 ms, faster than 55.20%

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
    int ans;
public:
    int diameterOfBinaryTree(TreeNode* root) {
        ans = 1;
        depth(root);
        return ans - 1;
    }
    int depth(TreeNode* node){
        if(node == NULL) return 0;
        int L = depth(node->left);
        int R = depth(node->right);
        ans = max(ans,L+R+1);
        return max(L,R) + 1;
    }
};

second.cpp

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third.cpp

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