Problem
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();
Solution
class Solution {
/** @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 head;
Random random;
public Solution(ListNode head) {
this.head = head;
random = new Random();
}
/** Returns a random node's value. */
public int getRandom() {
ListNode cur = head;
int res = cur.val, count = 1;
while (cur.next != null) {
cur = cur.next;
if (random.nextInt(count+1) == count) {
res = cur.val;
}
count++;
}
return res;
}
}
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