Q56 Random Pick with Blacklist - LeetCode

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Wed Feb 01 2023 06:15:04 GMT+0000 (Coordinated Universal Time)

Saved by @Ayush_dabas07

class Solution {
    HashMap<Integer,Integer> map;
    Random r;
    int top;
    public Solution(int n, int[] blacklist) {
        this.map = new HashMap<>();
        this.r = new Random();
        this.top = n - blacklist.length;

        HashSet<Integer> set = new HashSet<>();

        //adding all blacklist integers to set
        for(int i =0 ; i< blacklist.length;i++)
        set.add(blacklist[i]);

        n--;    //as range is 0 to n-1;

        for(int i = 0 ;i < blacklist.length ; i++){
            //if we have a bl element < top 
            if(blacklist[i] < top){
                
                //if the element to be mapped is also bl element move back
                while(set.contains(n))
                n--;

                //map to non bl element and move back
                map.put(blacklist[i],n);
                n--;
            }
        }

    }
    
    public int pick() {
        int ridx = r.nextInt(top);  //gives random b/w 0 to top-1, top is exclusive

        //if its a mapped element
        if(map.containsKey(ridx))
        return map.get(ridx);

        return ridx;
        
    }
}

/**
 * Your Solution object will be instantiated and called as such:
 * Solution obj = new Solution(n, blacklist);
 * int param_1 = obj.pick();
 */

https://leetcode.com/problems/random-pick-with-blacklist/

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