LeetCode 2199: Finding the Topic of Each Post Solution

Master LeetCode problem 2199 (Finding the Topic of Each Post), a hard challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.

2199. Finding the Topic of Each Post

Hard

Database

Problem Explanation

Explanation:

To solve this problem, we can use a graph-based approach. We can model the given posts and their topics as nodes and edges in a directed graph. Then, we can perform a topological sort to determine the topic of each post. The key idea is to construct the graph based on the provided post dependencies and then perform a topological sort to find the order of topics.

Algorithm:

Create a graph where each node represents a post and each directed edge (u, v) indicates that post u depends on post v.
Perform a topological sort on the graph to get the order of posts/topics.
Assign topics to posts based on the topological order obtained.

Time Complexity:

The time complexity of this approach is O(V + E), where V is the number of posts/topics and E is the number of dependencies between posts.

Space Complexity:

The space complexity is O(V + E) to store the graph and perform the topological sort.

Solution Code

import java.util.*;

class Solution {
    public int[] findOrder(int numCourses, int[][] prerequisites) {
        List<Integer>[] graph = new ArrayList[numCourses];
        int[] indegree = new int[numCourses];

        for (int i = 0; i < numCourses; i++) {
            graph[i] = new ArrayList<>();
        }

        for (int[] prerequisite : prerequisites) {
            int course = prerequisite[0];
            int prereq = prerequisite[1];
            graph[prereq].add(course);
            indegree[course]++;
        }

        Queue<Integer> queue = new LinkedList<>();
        for (int i = 0; i < numCourses; i++) {
            if (indegree[i] == 0) {
                queue.offer(i);
            }
        }

        int[] result = new int[numCourses];
        int index = 0;

        while (!queue.isEmpty()) {
            int node = queue.poll();
            result[index++] = node;
            for (int neighbor : graph[node]) {
                indegree[neighbor]--;
                if (indegree[neighbor] == 0) {
                    queue.offer(neighbor);
                }
            }
        }

        return index == numCourses ? result : new int[0];
    }
}

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