382. Linked List Random Node

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();
含义：给了我们一个链表，让我们随机返回一个节点。要求每个节点返回的概率是相同的

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    ArrayList<Integer> nums;
    HashMap<Integer, Integer> locs;
    java.util.Random rand = new java.util.Random();

    /**
     * Initialize your data structure here.
     */
    public RandomizedSet() {
        nums = new ArrayList<Integer>();
        locs = new HashMap<Integer, Integer>();
    }

    /**
     * Inserts a value to the set. Returns true if the set did not already contain the specified element.
     */
    public boolean insert(int val) {
        if (locs.containsKey(val)) return false;
        locs.put(val, nums.size());
        nums.add(val);
        return true;
    }

    /**
     * Removes a value from the set. Returns true if the set contained the specified element.
     */
    public boolean remove(int val) {
        if (!locs.containsKey(val)) return false;
        int loc = locs.get(val);
        if (loc < nums.size() - 1) { // not the last one than swap the last one with this val
            int lastone = nums.get(nums.size() - 1);
            nums.set(loc, lastone);
            locs.put(lastone, loc);
        }
        locs.remove(val);
        nums.remove(nums.size() - 1);
        return true;
    }

    /**
     * Get a random element from the set.
     */
    public int getRandom() {
        return nums.get(rand.nextInt(nums.size()));
    }
}

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