Counting elements

Array Hash Table

60.8% acceptance

Mar 31, 2026

168

Given an integer array arr, count how many elements x there are, such that x + 1 is also in arr. If there are duplicates in arr, count them separately.

Solution

Rust

Time O(n)

Space O(1)

LeetCode

solution.rs

use std::collections::HashSet;

impl Solution {
  pub fn count_elements(arr: Vec<i32>) -> i32 {
    let set: HashSet<i32> = arr.iter().copied().collect();
    arr.iter().filter(|&&x| set.contains(&(x + 1))).count() as i32
  }
}