/** * Definition for binary tree with next pointer. * struct TreeLinkNode { *  int val; *  TreeLinkNode *left, *right, *next; *  TreeLinkNode(int x) : val(x), left(NULL), right(NULL), next(NULL) {} * }; */class Solution {public:    void connect(TreeLinkNode *root) {        TreeLinkNode *head = NULL; //head of the next level        TreeLinkNode *prev = NULL; //the leading node on the next level        TreeLinkNode *cur = root;  //current node of current level        while (cur != NULL) {            while (cur != NULL) { //iterate on the current level                //left child                if (cur->left != NULL) {                    if (prev != NULL) {                        prev->next = cur->left;                    } else {                        head = cur->left;                    }                    prev = cur->left;                }                //right child                if (cur->right != NULL) {                    if (prev != NULL) {                        prev->next = cur->right;                    } else {                        head = cur->right;                    }                    prev = cur->right;                }                //move to next node                cur = cur->next;            }            //move to next level            cur = head;            head = NULL;            prev = NULL;        }    }};