This page explains Java solution to problem Insert Delete GetRandom Duplicates allowed
using HashMap Data Structure
algorithm.
Design a data structure that supports all following operations in average O(1)
time.
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.// 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();
package com.vc.hard;
import java.util.*;
class InsertDeleteGetRandomDuplicatesAllowed {
private HashMap<Integer, Set<Integer>> indexMap;
private List<Integer> list;
private Random random;
/** Initialize your data structure here. */
public InsertDeleteGetRandomDuplicatesAllowed() {
indexMap = new HashMap<>();
list = new ArrayList<>();
random = new 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 = !indexMap.containsKey(val);
Set<Integer> set = indexMap.getOrDefault(val, new HashSet<>());
set.add(list.size());
indexMap.put(val, set);
list.add(val);
return res;
}
/** Removes a value from the collection. Returns true if the collection contained the specified element. */
public boolean remove(int val) {
if(indexMap.containsKey(val)) {
int removeIndex = indexMap.get(val).iterator().next();
int swapElement = list.get(list.size() - 1);
indexMap.get(val).remove(removeIndex);
indexMap.get(swapElement).add(removeIndex);
indexMap.get(swapElement).remove(list.size() - 1);
if(indexMap.get(val).size() == 0) indexMap.remove(val);
list.set(removeIndex, swapElement);
list.remove(list.size() - 1);
return true;
}
return false;
}
/** Get a random element from the collection. */
public int getRandom() {
return list.get(random.nextInt(list.size()));
}
}
O(1)
O(N) Where
N is total number of elements in an input array