Course schedule ii

Depth-First Search Breadth-First Search Graph Theory Topological Sort

55.0% acceptance

Jan 12, 2026

11881

378

There are a total of numCourses courses you have to take, labeled from 0 to numCourses - 1. You are given an array prerequisites where prerequisites[i] = [ai, bi] indicates that you must take course bi first if you want to take course ai. For example, the pair [0, 1], indicates that to take course 0 you have to first take course 1. Return the ordering of courses you should take to finish all courses. If there are many valid answers, return any of them. If it is impossible to finish all courses, return an empty array.

Solution

Rust

Time O(n * m)

Space O(n * m)

LeetCode

solution.rs

impl Solution {
  pub fn find_order(num_courses: i32, prerequisites: Vec<Vec<i32>>) -> Vec<i32> {
    let n = num_courses as usize;
    let mut graph = vec![vec![]; n];
    let mut in_degree = vec![0; n];
    
    for prereq in prerequisites {
      graph[prereq[1] as usize].push(prereq[0] as usize);
      in_degree[prereq[0] as usize] += 1;
    }
    
    let mut queue = std::collections::VecDeque::new();
    for i in 0..n {
      if in_degree[i] == 0 {
        queue.push_back(i);
      }
    }
    
    let mut result = Vec::new();
    while let Some(node) = queue.pop_front() {
      result.push(node as i32);
      for &neighbor in &graph[node] {
        in_degree[neighbor] -= 1;
        if in_degree[neighbor] == 0 {
          queue.push_back(neighbor);
        }
      }
    }
    
    if result.len() == n { result } else { vec![] }
  }
}