[LeetCode] 398. Random Pick Index

QUESTION

1. Use a hash map <number, List<index>> to store the input array

2. Every time pick method invoke, get the list of index for given target number, get the random list index by rand.((max-min)+1)+min

public class Solution {
    Map<Integer, List<Integer>> map;
    Random rand;
    
    public Solution(int[] nums) {
        rand = new Random();
        map = new HashMap<Integer, List<Integer>>();
        for(int i = 0; i<nums.length; i++){
            if(!map.containsKey(nums[i])){
                map.put(nums[i], new ArrayList<Integer>());
            }
            map.get(nums[i]).add(i);
        }
    }
    
    public int pick(int target) {
        List<Integer> list = map.get(target);
        if(list.size() == 1){
            return list.get(0);
        }
        
        int listIndex = rand.nextInt(list.size());
        return list.get(listIndex);
    }
}

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

Without extra space

public class Solution {
    int[] nums;
    Random rand;
    
    public Solution(int[] nums) {
        this.rand = new Random();
        this.nums = nums;
    }
    
    public int pick(int target) {
        int res = -1;
        int upbound = 1;
        
        for(int i=0; i<nums.length; i++){
            if(nums[i] == target){
                if(rand.nextInt(upbound) == 0){
                    res = i;
                }
                upbound++;
            }
        }
        
        return res;
    }

}