in Leetcode questions, The tree node in C++ is defined as following:\n\n```\n/**\n * Definition for a binary tree node.\n * struct TreeNode {\n *     int val;\n *     TreeNode *left;\n *     TreeNode *right;\n *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}\n * };\n */\n```\n\nHow is the constructor ```  TreeNode(int x)``` implemented???

in Leetcode questions, The tree node in C++ is defined as following:\n\n\n/**\n * Definition for a binary tree node.\n * struct TreeNode {\n *     int val;\n * 