There are a total of n tasks you have to pick, labeled from 0 to n-1. Some tasks may have prerequisites, for example to pick task 0 you have to first pick task 1, which is expressed as a pair: [0, 1]
Given the total number of tasks and a list of prerequisite pairs, is it possible for you to finish all tasks?
Input: 2, [[1, 0]]
Explanation: There are a total of 2 tasks to pick. To pick task 1 you should have finished task 0. So it is possible.
Input: 2, [[1, 0], [0, 1]]
Explanation: There are a total of 2 tasks to pick. To pick task 1 you should have finished task 0, and to pick task 0 you should also have finished task 1. So it is impossible.
Input: 3, [[1, 0], [2, 1], [3, 2]]
Explanation: There are a total of 3 tasks to pick. To pick tasks 1 you should have finished task 0, and to pick task 2 you should have finished task 1 and to pick task 3 you should have finished task 2. So it is possible.
Asked In: Google, Twitter, Amazon and many more companies.
Solution: We can consider this problem as a graph (related to topological sorting) problem. All tasks are nodes of the graph and if task u is a prerequisite of task v, we will add a directed edge from node u to node v. Now, this problem is equivalent to detecting a cycle in the graph represented by prerequisites. If there is a cycle in the graph, then it is not possible to finish all tasks (because in that case there is no any topological order of tasks). Both BFS and DFS can be used to solve it.
Since pair is inconvenient for the implementation of graph algorithms, we first transform it to a graph. If task u is a prerequisite of task v, we will add a directed edge from node u to node v.
Prerequisite : Detect Cycle in a Directed Graph
Using DFS For DFS, it will first visit a node, then one neighbor of it, then one neighbor of this neighbor… and so on. If it meets a node which was visited in the current process of DFS visit, a cycle is detected and we will return false. Otherwise it will start from another unvisited node and repeat this process till all the nodes have been visited. Note that you should make two records: one is to record all the visited nodes and the other is to record the visited nodes in the current DFS visit.
The code is as follows. We use a vector visited to record all the visited nodes and another vector onpath to record the visited nodes of the current DFS visit. Once the current visit is finished, we reset the onpath value of the starting node to false.
Possible to finish all tasks
BFS can be used to solve it using the idea of topological sort. If topological sorting is possible, it means there is no cycle and it is possible to finish all the tasks.
BFS uses the indegrees of each node. We will first try to find a node with 0 indegree. If we fail to do so, there must be a cycle in the graph and we return false. Otherwise we have found one. We set its indegree to be -1 to prevent from visiting it again and reduce the indegrees of all its neighbors by 1. This process will be repeated for n (number of nodes) times. If we have not returned false, we will return true.
Possible to finish all tasks
- Find the ordering of tasks from given dependencies
- Sum of dependencies in a graph
- Construct the Rooted tree by using start and finish time of its DFS traversal
- Union-Find Algorithm | (Union By Rank and Find by Optimized Path Compression)
- Find the value of max(f(x)) - min(f(x)) for a given F(x)
- Find the value of f(n) / f(r) * f(n-r)
- Given two numbers a and b find all x such that a % x = b
- Find any pair with given GCD and LCM
- Program to find sum of 1 + x/2! + x^2/3! +...+x^n/(n+1)!
- Find minimum x such that (x % k) * (x / k) == n
- Find maximum value of x such that n! % (k^x) = 0
- Find the Nth term of the series 14, 28, 20, 40,.....
- Program to find the sum of the series (1/a + 2/a^2 + 3/a^3 + ... + n/a^n)
- Find the node whose xor with x gives minimum value
- Find the value of the function Y = (X^6 + X^2 + 9894845) % 971
If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to firstname.lastname@example.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.