# 398 Random Pick Index

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]], , , ]
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 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`.
 `````` 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 `````` ``````class Solution: def __init__(self, nums: List[int]): self.data = collections.defaultdict(list) for i, v in enumerate(nums): self.data[v].append(i) def pick(self, target: int) -> int: return random.choice(self.data[target]) # Your Solution object will be instantiated and called as such: # obj = Solution(nums) # param_1 = obj.pick(target) ''' Reservoir sampling (TLE) ''' class Solution: def __init__(self, nums: List[int]): self.data = nums def pick(self, target: int) -> int: count, idx = 0, 0 for i in range(len(self.data)): if self.data[i] == target: count += 1 if random.randint(0, count-1) == 0: idx = i return idx``````