Design a data structure that supports all following operations in average O(1) time.
insert(val): Inserts an item val to the set if not already present.remove(val): Removes an item val from the set if present.getRandom: Returns a random element from current set of elements. Each element must have the same probability of being returned.
Example:
// Init an empty set. RandomizedSet randomSet = new RandomizedSet(); // Inserts 1 to the set. Returns true as 1 was inserted successfully. randomSet.insert(1); // Returns false as 2 does not exist in the set. randomSet.remove(2); // Inserts 2 to the set, returns true. Set now contains [1,2]. randomSet.insert(2); // getRandom should return either 1 or 2 randomly. randomSet.getRandom(); // Removes 1 from the set, returns true. Set now contains [2]. randomSet.remove(1); // 2 was already in the set, so return false. randomSet.insert(2); // Since 2 is the only number in the set, getRandom always return 2. randomSet.getRandom();
1. 对于insert和remove，需要在O(1)的时间内查看该value是否存在，并insert/remove该value，因此考虑hash table。2. 对于getRandom，考虑用array来实现，因为：1）access每个元素的probablity相同；2）可以在O(1)时间内access任意元素。
1 class RandomizedSet {
2 public:
3 /** Initialize your data structure here. */
4 RandomizedSet() {
5
6 }
7
8 /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
9 bool insert(int val) {
10 if(map.find(val)==map.end()){ //val not presented in the set
11 map[val]=data.size();
12 data.push_back(val);
13 return true;
14 }
15
16 return false;
17 }
18
19 /** Removes a value from the set. Returns true if the set contained the specified element. */
20 bool remove(int val) {
21 if(map.find(val)!=map.end()){ // val presented in the set
22 int index = map[val];
23 map.erase(val);
24
25 //swap the index of the removed value with the last element's index
26 int lastIndex = data.size()-1;
27 if(index!=lastIndex){
28 map[data[lastIndex]] = index;
29 data[index]=data[lastIndex];
30 }
31 data.pop_back();
32 return true;
33 }
34
35 return false;
36 }
37
38 /** Get a random element from the set. */
39 int getRandom() {
40 return data[rand()%data.size()];
41 }
42 private:
43 vector<int> data; //data[index] = val
44 unordered_map<int,int> map; //map[val] = index
45 };
46
47 /**
48 * Your RandomizedSet object will be instantiated and called as such:
49 * RandomizedSet obj = new RandomizedSet();
50 * bool param_1 = obj.insert(val);
51 * bool param_2 = obj.remove(val);
52 * int param_3 = obj.getRandom();
53 */
Remove分兩步：
1. 把val从hash table里除去，hash table里的key是val，value是array里相应的index。
2.把val从array里除去，其中要除去的index是从hash table里查出的。具体做法是：将array[index]和array的最后的index互换，然后将array的size减一。因为array[index]和array[size-1]互换，hash table里的array[index] val也需要更新。
注意：当index是size-1时，不需要做：array[index]和array[size-1]互换，hash table里的array[index] val更新
方法二：
先将hash table更新，然后把hash table和array里的元素remove掉
1 class RandomizedSet {
2 public:
3 /** Initialize your data structure here. */
4 RandomizedSet() {
5
6 }
7
8 /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
9 bool insert(int val) {
10 if(map.find(val)!=map.end()){ //val already exists
11 return false;
12 }
13 array.push_back(val);
14 map[val]=array.size()-1;
15 return true;
16 }
17
18 /** Removes a value from the set. Returns true if the set contained the specified element. */
19 bool remove(int val) {
20 if(map.find(val)==map.end()){ //val not exist
21 return false;
22 }
23
24 int index=map[val];
25 map[array[array.size()-1]]=index;
26 array[index]=array[array.size()-1];
27 array.pop_back();
28 map.erase(val);
29
30 return true;
31 }
32
33 /** Get a random element from the set. */
34 int getRandom() {
35 if(array.size()>0){
36 int index=rand()%array.size();
37
38 return array[index];
39 }
40 return 0;
41 }
42 private:
43 unordered_map<int,int> map;
44 vector<int> array;
45 };
46
47 /**
48 * Your RandomizedSet object will be instantiated and called as such:
49 * RandomizedSet obj = new RandomizedSet();
50 * bool param_1 = obj.insert(val);
51 * bool param_2 = obj.remove(val);
52 * int param_3 = obj.getRandom();
53 */
来源：https://www.cnblogs.com/ruisha/p/9607758.html