382. Linked List Random Node

Medium (Trung bình) C++ 🔗 Xem trên LeetCode

📋 Đề Bài

Given a singly linked list, return a random node's value from the linked list. Each node must have the same probability of being chosen.

Implement the Solution class:

  • Solution(ListNode head) Initializes the object with the head of the singly-linked list head.
  • int getRandom() Chooses a node randomly from the list and returns its value. All the nodes of the list should be equally likely to be chosen.

 

Example 1:

Input
["Solution", "getRandom", "getRandom", "getRandom", "getRandom", "getRandom"]
[[[1, 2, 3]], [], [], [], [], []]
Output
[null, 1, 3, 2, 2, 3]

Explanation
Solution solution = new Solution([1, 2, 3]);
solution.getRandom(); // return 1
solution.getRandom(); // return 3
solution.getRandom(); // return 2
solution.getRandom(); // return 2
solution.getRandom(); // return 3
// getRandom() should return either 1, 2, or 3 randomly. Each element should have equal probability of returning.

 

Constraints:

  • The number of nodes in the linked list will be in the range [1, 104].
  • -104 <= Node.val <= 104
  • At most 104 calls will be made to getRandom.

 

Follow up:

  • What if the linked list is extremely large and its length is unknown to you?
  • Could you solve this efficiently without using extra space?

🧠 Thuật Toán & Kỹ Thuật

Linked List (Danh sách liên kết)
⏱️ Thời gian O(n)
💾 Không gian O(n)

💻 Lời Giải

C++ 0382-linked-list-random-node.cpp 
/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
private:
    vector<int> v;
    
public:
    Solution(ListNode* head) {
        while (head) {
            v.push_back(head->val);
            head = head->next;
        }
    }
    
    int getRandom() {
        return v[rand() % v.size()];
    }
};

/**
 * Your Solution object will be instantiated and called as such:
 * Solution* obj = new Solution(head);
 * int param_1 = obj->getRandom();
 */