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02_linked_list_random_node.cpp
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02_linked_list_random_node.cpp
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/*
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.
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?
Example:
// Init a singly linked list [1,2,3].
ListNode head = new ListNode(1);
head.next = new ListNode(2);
head.next.next = new ListNode(3);
Solution solution = new Solution(head);
// getRandom() should return either 1, 2, or 3 randomly. Each element should have equal probability of returning.
solution.getRandom();
*/
/**
* 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:
/** @param head The linked list's head.
Note that the head is guaranteed to be not null, so it contains at least one node. */
int length;
ListNode* start;
Solution(ListNode* head) {
length = 0;
start = head;
ListNode* temp = head;
while (temp) {
++length;
temp = temp->next;
}
}
/** Returns a random node's value. */
int getRandom() {
int idx = rand() % length;
ListNode* temp = start;
while (idx--)
temp = temp->next;
return temp->val;
}
};
/**
* Your Solution object will be instantiated and called as such:
* Solution* obj = new Solution(head);
* int param_1 = obj->getRandom();
*
*
// Reservoir Sampling method (Algorithm R by Alan Waterman)
/**
* 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:
/** @param head The linked list's head.
Note that the head is guaranteed to be not null, so it contains at least one node. */
ListNode* start;
Solution(ListNode* head) {
start = head;
}
/** Returns a random node's value. */
int getRandom() {
ListNode* temp = start;
int scope = 1, currVal = 0;
while (temp) {
if (((double)rand() / (RAND_MAX)) < 1.0 / scope)
currVal = temp->val;
temp = temp->next;
++scope;
}
return currVal;
}
};
/**
* Your Solution object will be instantiated and called as such:
* Solution* obj = new Solution(head);
* int param_1 = obj->getRandom();
*/