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382.java
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382.java
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__________________________________________________________________________________________________
sample 51 ms submission
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
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. */
int size = 0;
Random rand ;
ListNode head2;
public Solution(ListNode head) {
int count = 0;
ListNode curr = head;
while(curr !=null) {
count++;
curr = curr.next;
}
size = count;
rand = new Random();
head2 = head;
}
// int size;
/** Returns a random node's value. */
public int getRandom() {
ListNode curr = head2;
int r = rand.nextInt(size);
for (int i=0; i<r; i++) {
if (curr!= null){
curr = curr.next;
}
}
return curr.val;
}
}
/**
* Your Solution object will be instantiated and called as such:
* Solution obj = new Solution(head);
* int param_1 = obj.getRandom();
*/
__________________________________________________________________________________________________
sample 38244 kb submission
/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) { val = x; }
* }
*/
/*
水塘抽样算法
Problem:
Choose k entries from n numbers. Make sure each number is selected with the probability of k/n
Basic idea:
Choose 1, 2, 3, ..., k first and put them into the reservoir.
//下面的k+1表示第k+1个数字
For k+1, pick it with a probability of k/(k+1), and randomly replace a number in the reservoir.
For k+i, pick it with a probability of k/(k+i), and randomly replace a number in the reservoir.
Repeat until k+i reaches n
*/
class Solution {
private ListNode head;
private Random rand;
/** @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;
this.rand = new Random();
}
/** Returns a random node's value. */
public int getRandom() {
int k = 1;
ListNode node = this.head;
int i = 0;
ArrayList<Integer> reservoir = new ArrayList<Integer>();
//先把前k个放进水塘
while (i < k && node != null) {
reservoir.add(node.val);
node = node.next;
i++;
}
// i++; // i == k => i == k+1 这样i就代表了现在已经处理过的总共数字个位
i = 1;
while (node != null) {
//更换水塘里的数字的概率要是 k/(现在处理过的数字总数),所以因为i是从0开始,所以概率为从0
// 到i的数当中选0 到k-1的数字,rand.nextInt(i) < k的概率是k/(现在处理过的数字总数)
if (rand.nextInt(k+i) == i) {
reservoir.set(rand.nextInt(k), node.val);
}
i++;
node = node.next;
}
return reservoir.get(0);// or return reservoir when k > 1;
}
}
/**
* Your Solution object will be instantiated and called as such:
* Solution obj = new Solution(head);
* int param_1 = obj.getRandom();
*/
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