Populating Next Right Pointers in Each Node、Populating Next Right Pointers in Each Node II

Populating Next Right Pointers in Each Node:
Given a binary tree

    struct TreeLinkNode {
      TreeLinkNode *left;
      TreeLinkNode *right;
      TreeLinkNode *next;
    }

Populate each next pointer to point to its next right node. If there is no next right node, the next pointer should be set to  NULL .
Initially, all next pointers are set to  NULL .
Note:
You may only use constant extra space.
You may assume that it is a perfect binary tree (ie, all leaves are at the same level, and every parent has two children).
For example, Given the following perfect binary tree,

         1
       /  \
      2    3
     / \  / \
    4  5  6  7

After calling your function, the tree should look like:

         1 -> NULL
       /  \
      2 -> 3 -> NULL
     / \  / \
    4->5->6->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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        
    	TreeLinkNode* head=root;
		while(head)
		{
			TreeLinkNode guard(-1);
			TreeLinkNode* tail=&guard;
			while(head)
			{
				if( head->left )
				{
					tail->next=head->left;
					tail=tail->next;
				}
				if ( head->right )
				{
					tail->next=head->right;
					tail=tail->next;
				}
				head=head->next;
			}
			head=guard.next;
		}
    }
};