Q54 Insert Delete GetRandom O(1) - LeetCode

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Tue Jan 31 2023 10:14:17 GMT+0000 (Coordinated Universal Time)

Saved by @Ayush_dabas07

class RandomizedSet {
    
    HashMap<Integer,Integer>map;    //to store val , index
    ArrayList<Integer> arr; //to store values and getting random in O(1)
    
    Random r;

    public RandomizedSet() {
     this.map = new HashMap<>();
     this.arr = new ArrayList<>();
     

     this.r = new Random();
        
    }
    
    public boolean insert(int val) {
        
        if(this.map.containsKey(val)) return false;  //if already present
        
        this.map.put(val,arr.size());
        this.arr.add(val);
        

        return true;
    }
    
    public boolean remove(int val) {
        if(this.map.containsKey(val)==false) return false;
        
        //removing from AL in o(1)
        int idx = this.map.get(val);         //get index in O(1)
        this.map.remove(val);

        //if element to be removed is already at arr.size()-1 index
        if(arr.size()-1 == idx)
        arr.remove(arr.size()-1);

        else{
        //swap with arr.size()-1 element as traversing and getting the element will make it O(n)
        int current_value = this.arr.get(arr.size()-1);
        Collections.swap(this.arr, idx, arr.size()-1);
        
        //now remove latest element which is O(1) and also update index of that element
        // in HM
        this.arr.remove(arr.size()-1);
        this.map.put(current_value,idx); //update swapped value index;

        }

        return true;

    }
    
    public int getRandom() {
        //get random index from arraylist
        
        //this will give a random number from 0 to list.size()-1
        int idx = r.nextInt(arr.size());  

        int val = arr.get(idx);

        return val;
    }
}

/**
 * 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();
 */

https://leetcode.com/problems/insert-delete-getrandom-o1/

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