138. Copy List with Random Pointer

A linked list is given such that each node contains an additional random pointer which could point to any node in the list or null.

Return a deep copy of the list.

常规解法：

必须先遍历一遍复制single linked list。遍历过程中建立一个1对1的map，copy的每个node都是以之前node为key的pair的value。这样在第二次遍历的时候，只需要将random赋值就好。

public RandomListNode CopyRandomList(RandomListNode head) {
        if(head == null) return null;
        RandomListNode nextNode = null;
        var sentinel = new RandomListNode(-1);
        sentinel.next = head;
        var sentinel2 = new RandomListNode(-1);
        var dummy = sentinel2;
        var hashtable  = new Dictionary<RandomListNode,RandomListNode>();
        //copy;
        while(head != null)
        {
            var newCopy = new RandomListNode(head.label);
            hashtable.Add(head,newCopy);
            sentinel2.next = newCopy;
            head = head.next;
            sentinel2 = sentinel2.next;
        }
        //copy random
        var newHead = sentinel.next;
        var newListHead =  dummy.next;
        while(newHead != null)
        {
            var random = newHead.random;
            newListHead.random =(random==null)?null: hashtable[random];
            newHead = newHead.next;
            newListHead = newListHead.next;
        }
       
        return dummy.next;
    }

网上有大神http://fisherlei.blogspot.com/2013/11/leetcode-copy-list-with-random-pointer.html 给出了很巧妙的方法。

public RandomListNode CopyRandomList(RandomListNode head) {
        if(head == null) return null;
        RandomListNode nextNode = null;
        var sentinel = new RandomListNode(-1);
        sentinel.next = head;
        //copy;
        while(head != null)
        {
            var newCopy = new RandomListNode(head.label);
            nextNode = head.next;
            head.next = newCopy;
            newCopy.next = nextNode;
            head = nextNode;
        }
        //copy random
        var newHead = sentinel.next;
        while(newHead != null)
        {
            var random = newHead.random;
            var follow = newHead.next;
            follow.random =(random==null)? null:random.next;
            newHead = follow.next;
        }
        //split into 2 lists
        var legacy = sentinel.next;
        var pilot = new RandomListNode(-1);
        var dummy = pilot;
        while(legacy != null)
        {
            pilot.next = legacy.next;
            pilot =  pilot.next;
            legacy.next = legacy.next.next ;
            legacy = legacy.next;
        }
        return dummy.next;
    }