항해99 2주차 - 코스스케쥴

Jang Seok Woo·2022년 1월 23일

알고리즘

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Today I learned
2022/01/20

회고록

1/20

항해 99, 알고리즘 1주차

교재 : 파이썬 알고리즘 인터뷰

12장 그래프

1. 이론

https://velog.io/@jsw4215/%ED%95%AD%ED%95%B499-2%EC%A3%BC%EC%B0%A8-%EA%B7%B8%EB%9E%98%ED%94%84DFSBFS-%EC%9D%B4%EB%A1%A0-%EC%A0%95%EB%A6%AC

2. 문제

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 true if you can finish all courses. Otherwise, return false.

Example 1:

Input: numCourses = 2, prerequisites = [[1,0]]
Output: true
Explanation: There are a total of 2 courses to take.
To take course 1 you should have finished course 0. So it is possible.
Example 2:

Input: numCourses = 2, prerequisites = [[1,0],[0,1]]
Output: false
Explanation: There are a total of 2 courses to take.
To take course 1 you should have finished course 0, and to take course 0 you should also have finished course 1. So it is impossible.

Constraints:

1 <= numCourses <= 105
0 <= prerequisites.length <= 5000
prerequisites[i].length == 2
0 <= ai, bi < numCourses
All the pairs prerequisites[i] are unique.

https://leetcode.com/problems/course-schedule/

3. MySol

Recursive DFS

from typing import List
import collections


class Solution:
    def canFinish(self, numCourses: int, prerequisites: List[List[int]]) -> bool:
        graph = collections.defaultdict(list)
        for x, y in prerequisites:
            graph[x].append(y)

        traced = set()

        def dfs(i):
            if i in traced:
                return False

            traced.add(i)
            for y in graph[i]:
                if not dfs(y):
                    return False
            traced.remove(i)


            return True


        for x in list(graph):
            if not dfs(x):
                return False


        return True


if __name__ == '__main__':

    courses = [[0,1],[1,0]]
    num = 3

    sol = Solution()

    result = sol.canFinish(3, courses)

    print(f'result : {result}')