快排非递归

private static void nonRec_quickSort(int[] a,int start,int end) {
        LinkedList stack = new LinkedList();  //用栈模拟
        if(start < end) {
            stack.push(end);
            stack.push(start);
            while(!stack.isEmpty()) {
                int l = stack.pop();
                int r = stack.pop();
                int index = partition(a, l, r);
                if(l < index - 1) {
                    stack.push(index-1);
                    stack.push(l);
                }
                if(r > index + 1) {
                    stack.push(r);
                    stack.push(index+1);
                }
            }
        }
    }
    
    private static int partition(int[] a, int start, int end) {
        int pivot = a[start];
        while(start < end) {
            while(start < end && a[end] >= pivot)
                end--;
            a[start] = a[end];
            while(start < end && a[start] <= pivot)
                start++;
            a[end] = a[start];
        }
        a[start] = pivot;
        return start;
    }