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:

- `1 <= nums.length <= 2 * 104`
- `-231 <= nums[i] <= 231 - 1`
- `target` is an integer from `nums`.
- At most `104` calls will be made to `pick`.