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.

Solution1：Two pass(copy + random) Hashmap

思路：
(1)先处理元素本值复制部分: 遍历原list(nodes_src)，对应建立（复制）一个新的list(nodes_copy)，节点分别复制；
遍历过程中，建立一个map(node_src -> node_copy)，用来后续处理random
(2)random部分的复制： 同步遍历list_src 和 list_copy, node_copy的random为 其(node_copy)对应的node_src的random对应(src->映射map->copy)的node_copy
实现1a: 建立新list的时候直接先连起来，更快些
(实现1b: 不先连，在第二次遍历的时候通过get(node_src->node_copy)来找node_copy再连接，代码看起来简洁，但速度慢)

Time Complexity: O(N) Space Complexity: O(N)

Solution2：依次在旁边复制 连好random 再剥离

思路：Solution1中用hashmap是为了解决random时的node_src->node_copy的对应问题，是因为否则在遍历后我们无法访问到该copy中的random结点。
方法二先将node_copy在原list中复制，-> src -> [copy] -> src -> [copy] -> src -> [copy] -> ...，
这样一来在遍历的过程中，因为node_copy和node_src在同一list中，node_copy(即cur_src.next).random 就是cur_src.random.next(找到random对应的copy结点);
最后再extract the copy list 即可
Time Complexity: O(N) Space Complexity: O(1)

Solution1a Code：

public class Solution {
    public RandomListNode copyRandomList(RandomListNode src_head) {
        if(src_head == null) return null;

        HashMap node_map = new HashMap<>();

        // first traversal
        // (1)build a new node list by deeply copying from node_src
        // (2)build node_map mapping node_src -> node_copy for every node_src
        RandomListNode dummy_copy = new RandomListNode(0);
        RandomListNode cur_src = src_head;
        RandomListNode prev_copy = dummy_copy;
        while(cur_src != null) {
            RandomListNode node_copy = new RandomListNode(cur_src.label);
            prev_copy.next = node_copy;

            node_map.put(cur_src, node_copy);

            cur_src = cur_src.next;
            prev_copy = node_copy;
        }

        // second traversal
        // to add random ptr for each node_copy, by checking the map about which node_copy
        // (by corresponding node_src random relationship) that every node_copy.random should points to
        cur_src = src_head;
        RandomListNode cur_copy = dummy_copy.next;
        while(cur_src != null) {
            cur_copy.random = node_map.get(cur_src.random);
            cur_src = cur_src.next;
            cur_copy = cur_copy.next;
        }

        return dummy_copy.next;
    }
}

Solution1b Code：

public class Solution {
    public RandomListNode copyRandomList(RandomListNode src_head) {
        if(src_head == null) return null;

        HashMap node_map = new HashMap<>();

        // first traversal
        // build node_map mapping node_src -> node_copy for every node_src
        RandomListNode cur_src = src_head;
        while(cur_src != null) {
            RandomListNode node_copy = new RandomListNode(cur_src.label);
            node_map.put(cur_src, node_copy);
            cur_src = cur_src.next;
        }

        // second traversal
        // to add random ptr for each node_copy, by checking the map about which node_copy; and to link copy_nodes also by getting from map
        // (by corresponding node_src random relationship) that every node_copy.random should points to
        cur_src = src_head;
        while(cur_src != null) {
            node_map.get(cur_src).next = node_map.get(cur_src.next);
            node_map.get(cur_src).random = node_map.get(cur_src.random);
            cur_src = cur_src.next;
        }

        return node_map.get(src_head);
    }
}

Solution2 Code：

public class Solution {
    public RandomListNode copyRandomList(RandomListNode src_head) {
        if(src_head == null) return null;

        // First traversal: Make copy of each node
        // -> src -> [copy] -> src -> [copy] -> src -> [copy] -> ...
        RandomListNode cur_src = src_head;
        RandomListNode cur_src_next_save;
        while(cur_src != null) {
            cur_src_next_save = cur_src.next;
            
            RandomListNode node_copy = new RandomListNode(cur_src.label);
            cur_src.next = node_copy;
            node_copy.next = cur_src_next_save;
            
            cur_src = cur_src_next_save;
        }
        
        // Second traversal: assign random pointers for the copy nodes
        cur_src = src_head;
        while(cur_src != null) {
            if(cur_src.random != null) {
                cur_src.next.random = cur_src.random.next;
            }
            cur_src = cur_src.next.next;
        }
        
        // Third traversal: restore the original list, and extract the copy list.
        cur_src = src_head;
        RandomListNode dummy_copy = new RandomListNode(0);
        RandomListNode cur_copy = dummy_copy; 
        
        while(cur_src != null) {
            // extract the copy
            cur_copy.next = cur_src.next;
            cur_copy = cur_copy.next;
            
            // restore the original list
            cur_src.next = cur_src.next.next;
            cur_src = cur_src.next;
            
        }
        return dummy_copy.next; 
    }
}