Problem_Random Fetch - xwu36/LeetCode GitHub Wiki

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();

for deleting step, swapping the last element and the cur element

class RandomizedSet {
    List<Integer> list = new ArrayList<>();
    Map<Integer, Integer> map = new HashMap<>();
    /** 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.get(val) != null)
            return false;
        list.add(val);
        map.put(val, list.size() - 1);
        return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if(map.get(val) == null)
            return false;
        int last = list.get(list.size() - 1);
        int pl = map.get(val);
        map.put(last, pl);
        list.set(pl, last);
        list.remove(list.size() - 1);
        map.remove(val);
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        Integer pl = (int)(Math.random() * list.size());
        return list.get(pl);
    }
}

/**
 * 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();
 */

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();

Similar algorithm as the first problem.
use Map<Integer, Set<Integer>> data structure

class RandomizedCollection {
    List<Integer> list = new ArrayList<>();
    Map<Integer, Set<Integer>> map = new HashMap<>();
    /** Initialize your data structure here. */
    public RandomizedCollection() {
        
    }
    
    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    public boolean insert(int val) {
        boolean flag = !(map.get(val) != null && map.get(val).size() != 0);
        if(map.get(val) == null)
            map.put(val, new HashSet<Integer>());
        list.add(val);
        map.get(val).add(list.size() - 1);
        return flag;
    }
    
    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    public boolean remove(int val) {
        if(map.get(val) == null || map.get(val).size() == 0)
            return false;
        int pl = map.get(val).iterator().next();
        map.get(val).remove(pl);
        if(pl == list.size() - 1){
            list.remove(list.size() - 1);
            return true;
        }
        int last = list.get(list.size() - 1);
        list.set(pl, last);
        map.get(last).remove(list.size() - 1);
        map.get(last).add(pl);
        list.remove(list.size() - 1);
        return true;
    }
    
    /** Get a random element from the collection. */
    public int getRandom() {
        int pl = (int)(Math.random() * list.size());
        return list.get(pl);
    }
}

/**
 * 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();
 */