LC.P382[链表随机节点]

方法一：模拟

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/**
 * 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 {

    List<Integer> list;
    Random random;

    public Solution(ListNode head) {
        list = new ArrayList<>();
        random = new Random();
        while (head != null) {
            list.add(head.val);
            head = head.next;
        }
    }

    public int getRandom() {
        return list.get(random.nextInt(list.size()));
    }
}

/**
 * Your Solution object will be instantiated and called as such:
 * Solution obj = new Solution(head);
 * int param_1 = obj.getRandom();
 */
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(n)$

方法二：蓄水池抽样

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/**
 * 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 {

    ListNode head;
    Random random;

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

    public int getRandom() {
        int i = 1, ans = 0;
        for (ListNode cur = head; cur != null; cur = cur.next) {
            // 1 / i 的概率选中
            if (random.nextInt(i++) == 0) {
                ans = cur.val;
            }
        }
        return ans;
    }
}

/**
 * Your Solution object will be instantiated and called as such:
 * Solution obj = new Solution(head);
 * int param_1 = obj.getRandom();
 */
  • 时间复杂度：$O(n)$
  • 空间复杂度：$O(1)$