Linked List Random Node (M)
題目
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();
題意
思路
最簡單的方法是直接遍歷鏈表，把值全部存到List中，最后隨機下標即可，
官方解答還有一種神奇的水塘采樣方法，
代碼實作
Java
List存盤
class Solution {
private List<Integer> list = new ArrayList<>();
private Random random = new Random();
/** @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) {
while (head != null) {
list.add(head.val);
head = head.next;
}
}
/** Returns a random node's value. */
public int getRandom() {
int index = random.nextInt(list.size());
return list.get(index);
}
}
水塘采樣
class Solution {
private ListNode head;
/** @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() {
ListNode chosen = null;
int count = 1;
ListNode cur = head;
while (cur != null) {
if (Math.random() < 1.0 / count) {
chosen = cur;
}
count++;
cur = cur.next;
}
return chosen.val;
}
}
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