RandomPickIndex398.java

/**
 * 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);
 */

public class RandomPickIndex398 {

    class Solution {
        private Map<Integer, List<Integer>> map = new HashMap<>();
        private Random rand = new Random();
        public Solution(int[] nums) {
            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) {
            int idx = rand.nextInt(map.get(target).size());
            return map.get(target).get(idx);
        }
    }


    class Solution2 {
        private int[] nums;
        private Random random;
        
        public Solution2(int[] nums) {
            this.nums = nums;
            this.random = new Random();
        }
        
        public int pick(int target) {
            int count = 0;
            int res = -1;
            for (int i = 0; i < nums.length; i++) {
                if (nums[i] == target) {
                    count++;
                    int rand = random.nextInt(count) + 1;
                    if (rand == count) {
                        res = i;
                    }
                }
            }
            return res;
        }
    }

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

}