store the data from index 1 to n instead of normally 0 to n-1(a[1....n],then use std::sort(a+1,a+n);actually the last elements a[n] is not sort in qsort,so wa
hope this will be useful for u,try the following data
/****************
4
0 1
-1 2
-2 3
-3 4
************/