Skip to main content

MAY-29 2020 Challenge

Course Schedule


 There are a total of numCourses courses you have to take, labeled from 0 to numCourses-1.

 Some courses may have prerequisites, for example to take course 0 you have to first take course 1, which is expressed as a pair: [0,1]

 Given the total number of courses and a list of prerequisite pairs, is it possible for you to finish all courses?
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:


The input prerequisites is a graph represented by a list of edges, not adjacency matrices. Read more about how a graph is represented.
You may assume that there are no duplicate edges in the input prerequisites.
1 <= numCourses <= 10^5

Solution in jAvA:



class Solution {
    public boolean canFinish(int numCourses, int[][] prerequisites) {
        if (numCourses <= 0) {
            return true;
        }
        
        if (prerequisites == null || prerequisites.length == 0) {
            return true;
        }
        
        // First transform the edge list to adj. list
        Map<Integer, List<Integer>> adjList = new HashMap<>();
        for (int[] edge : prerequisites) {
            if (adjList.containsKey(edge[0])) {
                List<Integer> neighbors = adjList.get(edge[0]);
                neighbors.add(edge[1]);
                adjList.put(edge[0], neighbors);
            } else {
                List<Integer> neighbors = new ArrayList<Integer>();
                neighbors.add(edge[1]);
                adjList.put(edge[0], neighbors);
            }
        }
        
        int[] visited = new int[numCourses];
        // Check if the graph contains a circle, if yes, return false.
        for (int i = 0; i < numCourses; i++) {
            if (hasCircles(i, visited, adjList)) {
                return false;
            }
        }
        
        return true;
    }
    
    private boolean hasCircles(int vertexId, int[] visited, Map<Integer, List<Integer>> adjList) {
        if (visited[vertexId] == -1) {
            return true;
        }
        
        if (visited[vertexId] == 1) {
            return false;
        }
        
        visited[vertexId] = -1;
        
        List<Integer> neighbors = adjList.get(vertexId);
        if (neighbors != null) {
            for (int neighbor : neighbors) {
                if (hasCircles(neighbor, visited, adjList)) {
                    return true;
                }
            }
        }
        
        visited[vertexId] = 1;
        
        return false;
    }
}
Course Schedule

Labels:

Comments

Post a Comment

Popular posts from this blog

Leetcode 371. Sum of Two Integers. C++ / Java

371 .  Sum of Two Integers   Given two integers  a  and  b , return  the sum of the two integers without using the operators   +   and   - .   Example 1: Input: a = 1, b = 2 Output: 3 Example 2: Input: a = 2, b = 3 Output: 5   Constraints: -1000 <= a, b <= 1000 Solution :  C++ : class Solution { public: int getSum(int a, int b) { if (b==0) return a; int sum = a ^ b; int cr = (unsigned int) (a & b) << 1; return getSum(sum, cr); } }; Java :  class Solution { public int getSum(int a, int b) { while(b != 0){ int tmp = (a & b) << 1; a = a ^ b; b = tmp; } return a; } } Explaination :
Post a Comment
Read more

Leetcode 424. Longest Repeating Character Replacement. Python (Sliding Window)

  424 .  Longest Repeating Character Replacement You are given a string  s  and an integer  k . You can choose any character of the string and change it to any other uppercase English character. You can perform this operation at most  k  times. Return  the length of the longest substring containing the same letter you can get after performing the above operations .   Example 1: Input: s = "ABAB", k = 2 Output: 4 Explanation: Replace the two 'A's with two 'B's or vice versa. Example 2: Input: s = "AABABBA", k = 1 Output: 4 Explanation: Replace the one 'A' in the middle with 'B' and form "AABBBBA". The substring "BBBB" has the longest repeating letters, which is 4.   Constraints: 1 <= s.length <= 10 5 s  consists of only uppercase English letters. 0 <= k <= s.length Solution :  class Solution: def characterReplacement(self, s: str, k: int) -> int: hm = {} ans = 0 ...
Post a Comment
Read more

Leetcode 347. Top K Frequent Elements. Python (Bubble Sort)

Top K Frequent Elements Given an integer array  nums  and an integer  k , return  the   k   most frequent elements . You may return the answer in  any order .   Example 1: Input: nums = [1,1,1,2,2,3], k = 2 Output: [1,2] Example 2: Input: nums = [1], k = 1 Output: [1]   Constraints: 1 <= nums.length <= 10 5 -10 4  <= nums[i] <= 10 4 k  is in the range  [1, the number of unique elements in the array] . It is  guaranteed  that the answer is  unique .   Follow up:  Your algorithm's time complexity must be better than  O(n log n) , where n is the array's size. Solution : class Solution:     def topKFrequent(self, n: List[int], k: int) -> List[int]:                  # [1,1,1,2,2,3] &  k = 2                  f = [[] for i in range(len(n) + 1)]         # f = [...
Post a Comment
Read more