Leetcode: Random Pick Index

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);

Three types of answer:

1. Map Solution, O(N) memory, O(N) init, O(1) pick.

2. Like @dettier's Reservoir Sampling. O(1) init, O(1) memory, but O(N) to pick.

Reservior Sampling

 public class Solution {
     int[] arr;
     Random random;

     public Solution(int[] nums) {
         arr = nums;
         random = new Random();
     }

     public int pick(int target) {
         int count = 0;
         int index = 0;
         for (int i=0; i<arr.length; i++) {
             if (arr[i] == target) {
                 count++;
                 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);
  */

Map solution: MLE for big case

 public class Solution {

     public Solution(int[] nums) {
         for (int i=0; i<nums.length; i++) {
             int num = nums[i];
             if (!indexes.containsKey(num))
                 indexes.put(num, new ArrayList<Integer>());
             indexes.get(num).add(i);
         }
     }

     public int pick(int target) {
         List<Integer> indexes = this.indexes.get(target);
         int i = (int) (Math.random() * indexes.size());
         return indexes.get(i);
     }

     private Map<Integer, List<Integer>> indexes = new HashMap<>();
 }