计数排序问题

Question

public static void countingSort(Integer[] a, int n) {
        //TODO
        //COMPLETE THIS METHOD
        int[] counter = new int[n+1];
        int[] sorted = new int[a.length];
        Arrays.fill(counter,0);
        // fills counter array with each number count
        for ( int i = 0 ; i < a.length; i++){
            counter[a[i]] += 1;
        } // adds n-1 index + n index
        for (int i = 1; i < counter.length; i++) {
            counter[i] = (counter[i] + counter[i-1]);
        } // shifts array to the right
        for (int i = counter.length-1; i > 0; i--) {
            counter[i] = counter[i-1];
        } // fills sorted array with the sorted out counts
        for (int i = 0; i < a.length; i++ ){
            sorted[counter[a[i]]] = a[i];
            counter[a[i]]++;
        }
    }

When ran it throws an array out of bound exception in the body of the first for loop. Im having trouble seeing it, if anyone could help guide me, it'd be much appreciated.

Answer 1

When ran it throws an array out of bound exception in the body of the first for loop.

That would be this:

 for ( int i = 0; i < a.length; i++){ counter[a[i]] += 1; }

Im having trouble seeing it, if anyone could help guide me, it'd be much appreciated.

An ArrayIndexOutOfBoundsException in that loop can only mean that you are exceeding the bounds of a or those of counter . It is clear from inspection that you are not exceeding the bounds of a (though you could both eliminate that possibility and make the code a bit cleaner by switching to an enhanced for loop). The only remaining possibility is that you are exceeding the bounds of counter .

The approach you've implemented supports only non-negative integers in the input array, and requires the method invoker to pass an upper bound on the element values as the second parameter. If those constraints are not satisfied then an exception such as you observe will be thrown.

That is, if the exception is thrown where you say it is, then the problem is with the input.