LeetCode 0117 - Populating Next Right Pointers in Each Node II

     1
   /  \
  2    3
 / \    \
4   5    7

     1 -> NULL
   /  \
  2 -> 3 -> NULL
 / \    \
4-> 5 -> 7 -> NULL

/**
 * 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* cur = root;
        TreeLinkNode* nextLevelHead = NULL;
        TreeLinkNode* nextLevelPre = NULL;
        while(cur != NULL) {
            while(cur != NULL) {
                if(cur->left != NULL) {
                    if(nextLevelPre != NULL)
                        nextLevelPre->next = cur->left;
                    else
                        nextLevelHead = cur->left;
                    nextLevelPre = cur->left;
                }
                if(cur->right != NULL) {
                    if(nextLevelPre != NULL)
                        nextLevelPre->next = cur->right;
                    else
                        nextLevelHead = cur->right;
                    nextLevelPre = cur->right;
                }
                cur = cur->next;
            }
            cur = nextLevelHead;
            nextLevelHead = NULL;
            nextLevelPre = NULL;
        }        
    }
};