copy-list-with-random-pointer.cpp

// Time:  O(n)
// Space: O(1)

/**
 * Definition for singly-linked list with a random pointer.
 * struct RandomListNode {
 *     int label;
 *     RandomListNode *next, *random;
 *     RandomListNode(int x) : label(x), next(NULL), random(NULL) {}
 * };
 */
class Solution {
public:
    RandomListNode *copyRandomList(RandomListNode *head) {
        // Insert the copied node after the original one.
        for (auto *cur = head; cur; cur = cur->next->next) {
            auto *node = new RandomListNode(cur->label);
            node->next = cur->next;
            cur->next = node;
        }

        // Update random node.
        for (auto *cur = head; cur; cur = cur->next->next) {
            if (cur->random) {
                cur->next->random = cur->random->next;
            }
        }

        // Seperate the copied nodes from original ones.
        RandomListNode dummy(INT_MIN);
        for (auto *cur = head, *copy_cur = &dummy;
             cur;
             copy_cur = copy_cur->next, cur = cur->next) {
            copy_cur->next = cur->next;
            cur->next = cur->next->next;
        }

        return dummy.next;
    }
};