力扣：138.随机链表的复制

class Solution {
public:
    Node* copyRandomList(Node* head) {
        if ( !head ) return nullptr;
        Node *cur = head;
        while ( cur ) {
            Node *copy = new Node( cur->val ); // 复制节点的值
            copy->next = cur->next, cur->next = copy, cur = copy->next; // 把复制的节点插在两个节点之间
        }
        cur = head;
        while ( cur ) {
            Node *copy = cur->next;
            copy->random = cur->random ? cur->random->next : nullptr; // 把复制的节点的 random 指针建立起来
            cur = copy->next;
        }
        cur = head;
        Node *newHead = head->next, *copyCur = newHead;
        while ( cur ) {
            cur->next = copyCur->next, cur = cur->next; // 拆分两个链表
            if ( cur ) copyCur->next = cur->next, copyCur = copyCur->next;
        }
        return newHead;
    }
};

1. Node 有 3 个参数，val next 和 random
2. 为了空间复杂度最优，在原来链表的节点之后插入一个节点，复制这个节点的 val next 和 random