381. Insert Delete GetRandom O(1) - Duplicates allowed

Design a data structure that supports all following operations in average O(1) time.
Note: Duplicate elements are allowed.

insert(val)

: Inserts an item val to the collection.

remove(val)

: Removes an item val from the collection if present.

getRandom

: Returns a random element from current collection of elements.
The probability of each element being returned is linearly related to the number of same value the collection contains.

Example:

// Init an empty collection.
RandomizedCollection collection = new RandomizedCollection();

// Inserts 1 to the collection. Returns true as the collection did not contain 1.
collection.insert(1);

// Inserts another 1 to the collection. Returns false as the collection contained 1. Collection now contains [1,1].
collection.insert(1);

// Inserts 2 to the collection, returns true. Collection now contains [1,1,2].
collection.insert(2);

// getRandom should return 1 with the probability 2/3, and returns 2 with the probability 1/3.
collection.getRandom();

// Removes 1 from the collection, returns true. Collection now contains [1,2].
collection.remove(1);

// getRandom should return 1 and 2 both equally likely.
collection.getRandom();

这里的结构允许出现重复的数字,且在随机选取某个数时,概率相等.
仍然用hash表，但记录val在数组中的位置用set。
class RandomizedCollection {
map<int,set<int>> locs;//val to loc
vector<int> nums;
public:
/** Initialize your data structure here. */
RandomizedCollection() {

}

/** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
bool insert(int val) {
nums.push_back(val);
locs[val].insert(nums.size()-1);
if(locs[val].size()==1)
return 1;
else return 0;
}

/** Removes a value from the collection. Returns true if the collection contained the specified element. */
bool remove(int val) {
if(locs.find(val)==locs.end()) return 0;
int idx=*locs[val].begin();
int last=nums[nums.size()-1];
nums[idx]=last;

locs[val].erase(idx);//这里必须先删除一个才可以插入，如果last和val相等，先插入，会没有效果。这里调试了好久才找到错误
locs[last].insert(idx);
locs[last].erase(nums.size()-1);
nums.pop_back();

if(locs[val].size()==0) locs.erase(val);
return 1;

}

/** Get a random element from the collection. */
int getRandom() {
return nums[rand()%nums.size()];
}
};