Question
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();
Solution
User a map to track the position of the values in the list. Use a list to track the order of values.
Code
class RandomizedSet {
Map<Integer, Integer> map = new HashMap<>();
List<Integer> list = new ArrayList<>();
Random rand = new Random();
/** Initialize your data structure here. */
public RandomizedSet() {
}
/** 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;
int index = list.size();
list.add(val);
map.put(val, index);
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 index = map.get(val);
map.remove(val);
int lastIdx = list.size()-1;
if(index == lastIdx) {
list.remove(lastIdx);
}
else {
int lastVal = list.get(lastIdx);
map.put(lastVal, index);
list.set(index, lastVal);
list.remove(lastIdx);
}
return true;
}
/** Get a random element from the set. */
public int getRandom() {
return list.get(rand.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();
*/
Performance
TODO