Unique 3 digit even numbers

Array Hash Table Recursion Enumeration

69.2% acceptance

Feb 25, 2026

127

You are given an array of digits called digits. Your task is to determine the number of distinct three-digit even numbers that can be formed using these digits. Note: Each copy of a digit can only be used once per number, and there may not be leading zeros.

Solution

Rust

Time O(n³)

Space O(n)

LeetCode

solution.rs

impl Solution {
  pub fn total_numbers(digits: Vec<i32>) -> i32 {
    use std::collections::HashSet;
    let mut cnt = [0i32; 10];
    for &d in &digits { cnt[d as usize] += 1; }
    let mut seen = HashSet::new();
    let mut ans = 0;
    for a in 1..=9i32 {
      if cnt[a as usize] == 0 { continue; }
      cnt[a as usize] -= 1;
      for b in 0..=9i32 {
        if cnt[b as usize] == 0 { continue; }
        cnt[b as usize] -= 1;
        for c in (0..=8i32).step_by(2) {
          if cnt[c as usize] == 0 { continue; }
          let num = a * 100 + b * 10 + c;
          if seen.insert(num) { ans += 1; }
        }
        cnt[b as usize] += 1;
      }
      cnt[a as usize] += 1;
    }
    ans
  }
}