動機
真的就是用rand!!
Problem
Implement the RandomizedSet
class:
RandomizedSet()
Initializes theRandomizedSet
object.bool insert(int val)
Inserts an itemval
into the set if not present. Returnstrue
if the item was not present,false
otherwise.bool remove(int val)
Removes an itemval
from the set if present. Returnstrue
if the item was present,false
otherwise.int getRandom()
Returns a random element from the current set of elements (it's guaranteed that at least one element exists when this method is called). Each element must have the same probability of being returned.
You must implement the functions of the class such that each function works in average O(1)
time complexity.
Example 1:
Input[RandomizedSet, insert, remove, insert, getRandom, remove, insert, getRandom][[], [1], [2], [2], [], [1], [2], []]Output[null, true, false, true, 2, true, false, 2]ExplanationRandomizedSet randomizedSet = new RandomizedSet();randomizedSet.insert(1); // Inserts 1 to the set. Returns true as 1 was inserted successfully.randomizedSet.remove(2); // Returns false as 2 does not exist in the set.randomizedSet.insert(2); // Inserts 2 to the set, returns true. Set now contains [1,2].randomizedSet.getRandom(); // getRandom() should return either 1 or 2 randomly.randomizedSet.remove(1); // Removes 1 from the set, returns true. Set now contains [2].randomizedSet.insert(2); // 2 was already in the set, so return false.randomizedSet.getRandom(); // Since 2 is the only number in the set, getRandom() will always return 2.
Constraints:
-231 <= val <= 231 - 1
- At most
2 *
105
calls will be made toinsert
,remove
, andgetRandom
. - There will be at least one element in the data structure when
getRandom
is called.
Sol
#include <random>
class RandomizedSet {
private:
list<int> l;
unordered_map<int, list<int>::iterator> m;
std::mt19937_64 rng;
public:
/** Initialize your data structure here. */
RandomizedSet() {
std::random_device dev;
std::mt19937_64 rng2(dev());
rng = rng2;
}
/** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
bool insert(int val) {
bool ret = 0;
if (m.find(val) == m.end()) {
l.push_front(val);
m[val] = l.begin();
ret = 1;
}
return ret;
}
/** Removes a value from the set. Returns true if the set contained the specified element. */
bool remove(int val) {
bool ret = 0;
if (m.find(val) != m.end()) {
l.erase(m[val]);
m.erase(val);
ret = 1;
}
return ret;
}
/** Get a random element from the set. */
int getRandom() {
std::uniform_int_distribution<size_t> idDist(0, m.size() - 1);
auto item = m.begin();
std::advance( item, idDist(rng) );
return item->first;
}
};