Minimum rounds to complete all tasks

Array Hash Table Greedy Counting

63.2% acceptance

Feb 25, 2026

2849

You are given a 0-indexed integer array tasks, where tasks[i] represents the difficulty level of a task. In each round, you can complete either 2 or 3 tasks of the same difficulty level. Return the minimum rounds required to complete all the tasks, or -1 if it is not possible to complete all the tasks.

Solution

Rust

Time O(n)

Space O(n)

LeetCode

solution.rs

impl Solution {
  pub fn minimum_rounds(tasks: Vec<i32>) -> i32 {
    use std::collections::HashMap;
    let mut freq: HashMap<i32, i32> = HashMap::new();
    for t in tasks {
      *freq.entry(t).or_default() += 1;
    }
    let mut rounds = 0;
    for &count in freq.values() {
      if count == 1 {
        return -1;
      }
      // Minimum rounds: prefer groups of 3, if count%3==1 use one group of 2
      rounds += (count + 2) / 3;
    }
    rounds
  }
}