398. Random Pick Index

Description

Given an array of integers with possible duplicates, randomly output the index of a given target number. You can assume that the given target number must exist in the array.

Note:
The array size can be very large. Solution that uses too much extra space will not pass the judge.

Example:

int[] nums = new int[] {1,2,3,3,3};
Solution solution = new Solution(nums);

// pick(3) should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.
solution.pick(3);

// pick(1) should return 0\. Since in the array only nums[0] is equal to 1.
solution.pick(1);

Solution

HashMap + nextInt, constructor O(n), pick O(1), S(n)

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

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

Reservoir sampling, constructor O(1), pick O(n), S(n)

class Solution {
    private int[] nums;
    private Random random;
    
    public Solution(int[] nums) {
        this.nums = nums;
        random = new Random();
    }
    
    public int pick(int target) {
        int index = -1;
        int count = 0;
        
        for (int i = 0; i < nums.length; ++i) {
            if (nums[i] != target) {
                continue;
            }   
            
            if (random.nextInt(++count) == 0) {
                index = i;
            }
        }
        
        return index;
    }
}

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