LeetCode //C - 382. Linked List Random Node

382. Linked List Random Node

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 , 1 0 4 ] [1, 10^4] [1,104].
• − 1 0 4 < = N o d e . v a l < = 1 0 4 -10^4 <= Node.val <= 10^4 −104<=Node.val<=104
• At most 1 0 4 10^4 104 calls will be made to getRandom.

From: LeetCode

Link: 382. Linked List Random Node

---

Solution:

Ideas:

1. solutionCreate: Initializes the Solution object, stores the head of the linked list, and seeds the random number generator.

2. solutionGetRandom: Implements reservoir sampling to randomly select a node with equal probability. It iterates over the entire linked list and replaces the current result with a random chance, ensuring each node has an equal chance of being picked.

3. solutionFree: Frees the memory allocated for the Solution object.

Code:

// Definition for singly-linked list.
struct {
    int val;
    struct ListNode *next;
}ListNode;

typedef struct {
    struct ListNode* head;
} Solution;

Solution* solutionCreate(struct ListNode* head) {
    Solution* obj = (Solution*) malloc(sizeof(Solution));
    obj->head = head;
    srand(time(NULL)); // Initialize random number generator
    return obj;
}

int solutionGetRandom(Solution* obj) {
    struct ListNode* current = obj->head;
    int result = current->val;
    int i = 1;
    
    while (current != NULL) {
        // Pick a random number in range [0, i]
        if (rand() % i == 0) {
            result = current->val; // Replace the result with current node's value
        }
        current = current->next;
        i++;
    }
    return result;
}

void solutionFree(Solution* obj) {
    free(obj);
}

/**
 * Your Solution struct will be instantiated and called as such:
 * Solution* obj = solutionCreate(head);
 * int param_1 = solutionGetRandom(obj);
 * solutionFree(obj);
 */