Nuts & Bolts Problem

/**
 * public class NBCompare {
 *     public int cmp(String a, String b);
 * }
 * You can use compare.cmp(a, b) to compare nuts "a" and bolts "b",
 * if "a" is bigger than "b", it will return 1, else if they are equal,
 * it will return 0, else if "a" is smaller than "b", it will return -1.
 * When "a" is not a nut or "b" is not a bolt, it will return 2, which is not valid.
*/
public class Solution {
    /**
     * @param nuts: an array of integers
     * @param bolts: an array of integers
     * @param compare: a instance of Comparator
     * @return: nothing
     */
    public void sortNutsAndBolts(String[] nuts, String[] bolts, NBComparator compare) {
        // write your code here
        if (nuts == null || bolts == null || nuts.length != bolts.length || compare == null) {
            return;
        }
        quicksort(nuts, bolts, 0, nuts.length - 1, compare);
    }
    
    /**
     * @param nuts: an array of integers
     * @param bolts: an array of integers
     * @param start: the start position of the array
     * @param end: the end position of the array
     * @param compare: a instance of Comparator
     * @return: nothing
     */    
    public void quicksort(String[] nuts, String[] bolts, int start, int end, NBComparator compare) {
        if (start >= end) {
            return;
        }
        String nut = nuts[start];
        int index = partition(bolts, start, end, nut, compare);
        String bolt = bolts[index];
        partition(nuts, start, end, bolt, compare);
        quicksort(nuts, bolts, start, index - 1, compare);
        quicksort(nuts, bolts, index + 1, end, compare);
    }

    /**
     * @param arr: an array of integers
     * @param start: the start position of the array
     * @param end: the end position of the array
     * @param target: the pivot element to be compared to seperate the array
     * @param compare: a instance of Comparator
     * @return: the final index of the target after partition is done 
     */      
    public int partition(String[] arr, int start, int end, String target, NBComparator compare) {
        if (start >= end) {
            return start;
        }
        for (int i = start; i <= end - 1; i++) {
            String s = arr[i];
            if (compare.cmp(s, target) == -1 || compare.cmp(target, s) == 1) {
                swap(arr, i, start++);
            } else if(compare.cmp(s, target) == 0 || compare.cmp(target, s) == 0) {
                swap(arr, i--, end);
            }
        }
        swap(arr, start, end);
        return start;
    } 
    
    /**
     * @param arr: an array of integers
     * @param i: the start position of the array
     * @param j: the end position of the array
     */          
    public void swap(String[] arr, int i, int j) {
        String temp = arr[i];
        arr[i] = arr[j];
        arr[j] = temp;
    }
};