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[0] is equal to 1.
solution.pick(1);

Rust Solution

use rand::prelude::*;

struct Solution {
    rng: ThreadRng,
    nums: Vec<i32>,
}

impl Solution {
    fn new(nums: Vec<i32>) -> Self {
        let rng = thread_rng();
        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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