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
想法是利用上一层生成的链表，遍历这个链表，把这个链表的下一层节点连起来，形成新的链表。

var connect = function(root) {
    if (!root)
        return;
    var head = root;
    head.next=null;
    while (head) {
        var temp = head;
        if (head.left)
            head = head.left;
        else {
            head = null;
            break;
        }
        while (temp.next) {
            temp.left.next = temp.right;
            temp.right.next = temp.next.left;
            temp = temp.next;
        }
        temp.left.next = temp.right;
    }
};