Linked List Random Node Problem


Description

LeetCode Problem 382.

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 integer array nums.
  • 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 choosen.

Example 1:

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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, 10^4].
  • -10^4 <= Node.val <= 10^4
  • At most 10^4 calls will be made to getRandom.


Sample C++ Code

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/**
 * 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 {
public:
    ListNode* HeadNode;
    Solution(ListNode* head) {
       HeadNode = head;
    }
    int getRandom() {
        int res, len = 1;
        ListNode* x = HeadNode;
        while(x){
            if(rand() % len == 0){
                res = x->val;
            }
            len++;
            x = x->next;
        }
        return res;
    }
};

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




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