LeetCode[380] Insert Delete GetRandom O(1)

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 1 is the only number in the set, getRandom always return 1.

randomSet.getRandom();

用一个数组和一个哈希表，其中数组用来保存数字，哈希表用来建立每个数字和其在数组中的位置之间的映射，对于插入操作，我们先看这个数字是否已经在哈希表中存在，如果存在的话直接返回false，不存在的话，我们将其插入到数组的末尾，然后建立数字和其位置的映射。删除操作还是要先判断其是否在哈希表里，如果没有，直接返回false。由于哈希表的删除是常数时间的，而数组并不是，为了使数组删除也能常数级，我们实际上将要删除的数字和数组的最后一个数字调换个位置，然后修改对应的哈希表中的值，这样我们只需要删除数组的最后一个元素即可，保证了常数时间内的删除。而返回随机数对于数组来说就很简单了，我们只要随机生成一个位置，返回该位置上的数字即可。

class RandomizedSet {
public:
/** Initialize your data structure here. */
RandomizedSet() {

}

/** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
bool insert(int val) {
if (hash.count(val))
return false;
number.push_back(val);
hash[val] = number.size() - 1;
return true;
}

/** Removes a value from the set. Returns true if the set contained the specified element. */
bool remove(int val) {
if (!hash.count(val))
return false;
int last = number.back();
number[hash[val]] = last;
number.pop_back();
hash[last] = hash[val];
hash.erase(val);
return true;
}

/** Get a random element from the set. */
int getRandom() {
return number[rand() % number.size()];
}
private:
unordered_map<int, int> hash;
vector<int> number;
};

/**
* Your RandomizedSet object will be instantiated and called as such:
* RandomizedSet obj = new RandomizedSet();
* bool param_1 = obj.insert(val);
* bool param_2 = obj.remove(val);
* int param_3 = obj.getRandom();
*/