## 398. Random Pick Index

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 is equal to 1.
solution.pick(1);
```

## Rust Solution

``````use rand::prelude::*;

struct Solution {
nums: Vec<i32>,
}

impl Solution {
fn new(nums: Vec<i32>) -> Self {
Solution { rng, nums }
}
fn pick(&mut self, target: i32) -> i32 {
let n = self.nums.len();
let mut count = 0;
let mut res = 0;
for i in 0..n {
if self.nums[i] == target {
count += 1;
if self.rng.gen_range(0, count) == 0 {
res = i;
}
}
}
res as i32
}
}
``````

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