LeetCode – Course Schedule II (Java)

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This is an extension of Course Schedule. This time a valid sequence of courses is required as output.

Analysis

If we use the BFS solution of Course Schedule, a valid sequence can easily be recorded.

Java Solution

public int[] findOrder(int numCourses, int[][] prerequisites) {
    if(prerequisites == null){
        throw new IllegalArgumentException("illegal prerequisites array");
    }
 
    int len = prerequisites.length;
 
    //if there is no prerequisites, return a sequence of courses
    if(len == 0){
        int[] res = new int[numCourses];
        for(int m=0; m<numCourses; m++){
            res[m]=m;
        }
        return res;
    }
 
    //records the number of prerequisites each course (0,...,numCourses-1) requires
    int[] pCounter = new int[numCourses];
    for(int i=0; i<len; i++){
        pCounter[prerequisites[i][0]]++;
    }
 
    //stores courses that have no prerequisites
    LinkedList<Integer> queue = new LinkedList<Integer>();
    for(int i=0; i<numCourses; i++){
        if(pCounter[i]==0){
            queue.add(i);
        }
    }
 
    int numNoPre = queue.size();
 
    //initialize result
    int[] result = new int[numCourses];
    int j=0;
 
    while(!queue.isEmpty()){
        int c = queue.remove();
        result[j++]=c;
 
        for(int i=0; i<len; i++){
            if(prerequisites[i][1]==c){
                pCounter[prerequisites[i][0]]--;
                if(pCounter[prerequisites[i][0]]==0){
                    queue.add(prerequisites[i][0]);
                    numNoPre++;
                }
            }
 
        }
    }
 
    //return result
    if(numNoPre==numCourses){
        return result;
    }else{
        return new int[0];
    }
}

6 thoughts on “LeetCode – Course Schedule II (Java)”

  1. Use below collection :
    Set[] starts = new HashSet[numCourses];
    for (int i = 0; i < prerequisites.length ; i++) {
    starts[prerequisites[i][1]].add(prerequisites[i][0]);
    }

    to replace with iterating all the prerequisites
    for(int i=0; i<len; i++){
    if(prerequisites[i][1]==c){
    pCounter[prerequisites[i][0]]–;
    if(pCounter[prerequisites[i][0]]==0){
    queue.add(prerequisites[i][0]);
    numNoPre++;
    }
    }

    }

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