Sort

Quicksort

public int[] quickSort(int[] arr) {
    helper(arr,0,arr.length-1);
    return arr;
}

public void helper(int[] arr, int start, int end) {
    if(start>=end) return;
    int mid = partition(arr,start,end);
    helper(arr,start,mid-1);
    helper(arr,mid+1,end);
}


public int partition(int[] arr, int lo, int hi) {
    int pivot = arr[hi]; //pivot (Element to be placed at right position)
    int i = lo; //left of i is element smaller than pivot
    for (int j=lo; j<=hi; j++) { //element to be processed (include j)
        if (arr[j] <= pivot)
            swap(arr, i++ ,j);
    }
    return i-1; //return the pivot
}

public void swap(int[] arr, int i, int j) {
    int tmp = arr[i];
    arr[i] = arr[j];
    arr[j] = tmp;
}

Mergesort