382. Linked List Random Node

Description

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?

Solutions

Solution 1

Python3JavaC++Go

# 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: Optional[ListNode]):
        self.head = head

    def getRandom(self) -> int:
        n = ans = 0
        head = self.head
        while head:
            n += 1
            x = random.randint(1, n)
            if n == x:
                ans = head.val
            head = head.next
        return ans


# Your Solution object will be instantiated and called as such:
# obj = Solution(head)
# param_1 = obj.getRandom()

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    private ListNode head;
    private Random random = new Random();

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

    public int getRandom() {
        int ans = 0, n = 0;
        for (ListNode node = head; node != null; node = node.next) {
            ++n;
            int x = 1 + random.nextInt(n);
            if (n == x) {
                ans = node.val;
            }
        }
        return ans;
    }
}

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

    Solution(ListNode* head) {
        this->head = head;
    }

    int getRandom() {
        int n = 0, ans = 0;
        for (ListNode* node = head; node != nullptr; node = node->next) {
            n += 1;
            int x = 1 + rand() % n;
            if (n == x) ans = node->val;
        }
        return ans;
    }
};

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

/**
 * Definition for singly-linked list.
 * type ListNode struct {
 *     Val int
 *     Next *ListNode
 * }
 */
type Solution struct {
    head *ListNode
}

func Constructor(head *ListNode) Solution {
    return Solution{head}
}

func (this *Solution) GetRandom() int {
    n, ans := 0, 0
    for node := this.head; node != nil; node = node.Next {
        n++
        x := 1 + rand.Intn(n)
        if n == x {
            ans = node.Val
        }
    }
    return ans
}

/**
 * Your Solution object will be instantiated and called as such:
 * obj := Constructor(head);
 * param_1 := obj.GetRandom();
 */

382. Linked List Random Node

Description

Solutions

Solution 1

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