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382.linked-list-random-node.cpp
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108 lines (101 loc) · 2.68 KB
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// Tag: Linked List, Math, Reservoir Sampling, Randomized
// Time: O(N)
// Space: O(1)
// Ref: -
// Note: -
// 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?
//
//
/**
* 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 *h;
Solution(ListNode* head) {
h = head;
}
int getRandom() {
ListNode *res = h;
ListNode *cur = h->next;
int i = 2;
while (cur) {
if (rand() % i == 0) {
res = cur;
}
cur = cur->next;
i++;
}
return res->val;
}
};
class Solution {
public:
ListNode* h;
int count;
Solution(ListNode* head) {
h = head;
ListNode* cur = h;
count = 0;
while (cur) {
cur = cur->next;
count++;
}
}
int getRandom() {
int t = rand() % count + 1;
ListNode* cur = h;
for (int i = 1; i < t; i++) {
cur = cur->next;
}
return cur->val;
}
};
/**
* Your Solution object will be instantiated and called as such:
* Solution* obj = new Solution(head);
* int param_1 = obj->getRandom();
*/