## DEV Community

Abhishek Chaudhary

Posted on

# 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]], [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`.

SOLUTION:

``````import random

class Solution:

def __init__(self, nums: List[int]):
self.valIndex = {}
for i, num in enumerate(nums):
self.valIndex[num] = self.valIndex.get(num, []) + [i]

def pick(self, target: int) -> int:
return random.choice(self.valIndex[target])

# Your Solution object will be instantiated and called as such:
# obj = Solution(nums)
# param_1 = obj.pick(target)
``````