First unique even element

Array Hash Table Counting

66.3% acceptance

Mar 31, 2026

You are given an integer array nums. Return an integer denoting the first even integer (earliest by array index) that appears exactly once in nums. If no such integer exists, return -1. An integer x is considered even if it is divisible by 2.

Solution

Rust

Time O(n)

Space O(n)

LeetCode

solution.rs

use std::collections::HashMap;

impl Solution {
  pub fn first_unique_even(nums: Vec<i32>) -> i32 {
    let mut count = HashMap::new();
    for &x in &nums {
      if x % 2 == 0 {
        *count.entry(x).or_insert(0) += 1;
      }
    }
    for &x in &nums {
      if x % 2 == 0 && count[&x] == 1 {
        return x;
      }
    }
    -1
  }
}