LCR.P30[O(1)时间插入、删除和获取随机元素]

方法一：哈希表+List模拟

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
class RandomizedSet {

    Map<Integer, Integer> map;
    List<Integer> list;
    Random random;

    /**
     * Initialize your data structure here.
     */
    public RandomizedSet() {
        map = new HashMap<>();
        list = new ArrayList<>();
        random = new Random();
    }

    /**
     * Inserts a value to the set. Returns true if the set did not already contain the specified element.
     */
    public boolean insert(int val) {
        if (map.containsKey(val)) return false;
        map.put(val, list.size());
        list.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 (!map.containsKey(val)) return false;
        int size = list.size();
        int index = map.get(val);
        int lastVal = list.get(size - 1);
        map.put(lastVal, index);
        map.remove(val);
        list.set(index, lastVal);
        list.remove(size - 1);
        return true;
    }

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

/**
 * Your RandomizedSet object will be instantiated and called as such:
 * RandomizedSet obj = new RandomizedSet();
 * boolean param_1 = obj.insert(val);
 * boolean param_2 = obj.remove(val);
 * int param_3 = obj.getRandom();
 */
  • 时间复杂度：$O(1)$
  • 空间复杂度：$O(n)$