1365. How Many Numbers Are Smaller Than the Current Number

Input: nums = [8,1,2,2,3]
Output: [4,0,1,1,3]
Explanation: 
For nums[0]=8 there exist four smaller numbers than it (1, 2, 2 and 3). 
For nums[1]=1 does not exist any smaller number than it.
For nums[2]=2 there exist one smaller number than it (1). 
For nums[3]=2 there exist one smaller number than it (1). 
For nums[4]=3 there exist three smaller numbers than it (1, 2 and 2).

Input: nums = [6,5,4,8]
Output: [2,1,0,3]

Input: nums = [7,7,7,7]
Output: [0,0,0,0]

Rust Solution

struct Solution;

impl Solution {
    fn smaller_numbers_than_current(nums: Vec<i32>) -> Vec<i32> {
        let mut count = vec![0; 101];
        for &x in &nums {
            count[x as usize] += 1;
        }
        for i in 0..100 {
            count[i + 1] += count[i]
        }
        let mut res = vec![];
        for &x in &nums {
            let v = if x == 0 { 0 } else { count[(x - 1) as usize] };
            res.push(v)
        }
        res
    }
}

#[test]
fn test() {
    let nums = vec![8, 1, 2, 2, 3];
    let res = vec![4, 0, 1, 1, 3];
    assert_eq!(Solution::smaller_numbers_than_current(nums), res);
    let nums = vec![6, 5, 4, 8];
    let res = vec![2, 1, 0, 3];
    assert_eq!(Solution::smaller_numbers_than_current(nums), res);
    let nums = vec![7, 7, 7, 7];
    let res = vec![0, 0, 0, 0];
    assert_eq!(Solution::smaller_numbers_than_current(nums), res);
}