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();
The idea is to add a set to the hashMap to remember all the locations of a duplicated number.
public class RandomizedCollection {
HashMap<Integer, HashSet<Integer>> map;
ArrayList<Integer> arr;
java.util.Random random;
/** Initialize your data structure here. */
public RandomizedCollection() {
map = new HashMap<Integer, HashSet<Integer>>();
arr = new ArrayList<Integer>();
random = new java.util.Random();
}
/** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
public boolean insert(int val) {
boolean res = false;
if (!map.containsKey(val)) {
map.put(val, new HashSet<Integer>());
res = true;
}
arr.add(val);
map.get(val).add(arr.size()-1);
return res;
}
/** Removes a value from the collection. Returns true if the collection contained the specified element. */
public boolean remove(int val) {
if (!map.containsKey(val)) return false;
int lastItem = arr.get(arr.size()-1);
int index = arr.size()-1;
if (lastItem != val) {
HashSet<Integer> lastItemSet = map.get(lastItem);
index = map.get(val).iterator().next();
arr.set(index, lastItem);
lastItemSet.remove(arr.size()-1);
lastItemSet.add(index);
}
if (map.get(val).size() == 1) map.remove(val);
else map.get(val).remove(index);
arr.remove(arr.size()-1);
return true;
}
/** Get a random element from the collection. */
public int getRandom() {
return arr.get(random.nextInt(arr.size()));
}
}
/**
* Your RandomizedCollection object will be instantiated and called as such:
* RandomizedCollection obj = new RandomizedCollection();
* boolean param_1 = obj.insert(val);
* boolean param_2 = obj.remove(val);
* int param_3 = obj.getRandom();
*/