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382. Linked List Random Node 👍

  • Time: $O(n)$
  • Space: $O(1)$
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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. */
  Solution(ListNode* head) : head(head) {}

  /** Returns a random node's value. */
  int getRandom() {
    int res = -1;
    int i = 1;

    for (ListNode* curr = head; curr; curr = curr->next, ++i)
      if (rand() % i == 0)
        res = curr->val;

    return res;
  }

 private:
  ListNode* head;
};

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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.
   */
  public Solution(ListNode head) {
    this.head = head;
  }

  /** Returns a random node's value. */
  public int getRandom() {
    int res = -1;
    int i = 1;

    for (ListNode curr = head; curr != null; curr = curr.next, ++i)
      if (rand.nextInt(i) == i - 1)
        res = curr.val;

    return res;
  }

  private ListNode head;
  private Random rand = new Random();
}

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# Definition for singly-linked list.
# class ListNode:
#   def __init__(self, val=0, next=None):
#     self.val = val
#     self.next = next

class Solution:
  def __init__(self, head: ListNode | None):
    self.head = head

  def getRandom(self) -> int:
    res = -1
    i = 1
    curr = self.head

    while curr:
      if random.randint(0, i - 1) == 0:
        res = curr.val
      curr = curr.next
      i += 1

    return res