398 - Random Pick Index.

Medium

Given an integer array nums 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.

Implement the Solution class:

• Solution(int[] nums) Initializes the object with the array nums.

• int pick(int target) Picks a random index i from nums where nums[i] == target. If there are multiple valid i's, then each index should have an equal probability of returning.

Example 1:

Input

["Solution", "pick", "pick", "pick"]
[[[1, 2, 3, 3, 3]], [3], [1], [3]]

Output: null, 4, 0, 2

Explanation:

Solution solution = new Solution([1, 2, 3, 3, 3]);
solution.pick(3); // It should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.
solution.pick(1); // It should return 0. Since in the array only nums[0] is equal to 1.
solution.pick(3); // It should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.

Constraints:

• <code>1 <= nums.length <= 2 * 10⁴</code>

• <code>-2³¹<= numsi<= 2³¹ - 1</code>

• target is an integer from nums.

• At most <code>10⁴</code> calls will be made to pick.