README.md

1462. Course Schedule IV

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 ai first if you want to take course bi.

• For example, the pair [0, 1] indicates that you have to take course 0 before you can take course 1.

Prerequisites can also be indirect. If course a is a prerequisite of course b, and course b is a prerequisite of course c, then course a is a prerequisite of course c.

You are also given an array queries where queries[j] = [uj, vj]. For the jth query, you should answer whether course uj is a prerequisite of course vj or not.

Return a boolean array answer, where answer[j] is the answer to the jth query.

Example 1:

Input: numCourses = 2, prerequisites = [[1,0]], queries = [[0,1],[1,0]]
Output: [false,true]
Explanation: The pair [1, 0] indicates that you have to take course 1 before you can take course 0.
Course 0 is not a prerequisite of course 1, but the opposite is true.

Example 2:

Input: numCourses = 2, prerequisites = [], queries = [[1,0],[0,1]]
Output: [false,false]
Explanation: There are no prerequisites, and each course is independent.

Example 3:

Input: numCourses = 3, prerequisites = [[1,2],[1,0],[2,0]], queries = [[1,0],[1,2]]
Output: [true,true]

Constraints:

• 2 <= numCourses <= 100
• 0 <= prerequisites.length <= (numCourses * (numCourses - 1) / 2)
• prerequisites[i].length == 2
• 0 <= ai, bi <= numCourses - 1
• ai != bi
• All the pairs [ai, bi] are unique.
• The prerequisites graph has no cycles.
• 1 <= queries.length <= 104
• 0 <= ui, vi <= numCourses - 1
• ui != vi

Solutions (Rust)

1. Solution

impl Solution {
    pub fn check_if_prerequisite(
        num_courses: i32,
        prerequisites: Vec<Vec<i32>>,
        queries: Vec<Vec<i32>>,
    ) -> Vec<bool> {
        use std::collections::HashSet;

        let num_courses = num_courses as usize;
        let mut prerequisites_set = HashSet::new();
        let mut is_prerequisite = vec![vec![false; num_courses]; num_courses];
        let mut indegree = vec![0; num_courses];
        let mut stack = vec![];

        for prerequisite in &prerequisites {
            let (a, b) = (prerequisite[0] as usize, prerequisite[1] as usize);

            prerequisites_set.insert((a, b));
            is_prerequisite[a][b] = true;
            indegree[b] += 1;
        }

        for a in 0..num_courses {
            if indegree[a] == 0 {
                stack.push(a);
            }
        }

        while let Some(a) = stack.pop() {
            for b in 0..num_courses {
                if is_prerequisite[a][b] {
                    for c in 0..num_courses {
                        is_prerequisite[c][b] |= is_prerequisite[c][a];
                    }
                }

                if prerequisites_set.contains(&(a, b)) {
                    indegree[b] -= 1;
                    if indegree[b] == 0 {
                        stack.push(b);
                    }
                }
            }
        }

        queries
            .iter()
            .map(|query| is_prerequisite[query[0] as usize][query[1] as usize])
            .collect()
    }
}