PAT/PATBasic/1035.c

#include <stdio.h>
#include <stdlib.h>

int comp(const void *a, const void *b)
{
    return *(int*)a - *(int*)b;
}

int main()
{
    int N, origin[100], halfsort[100], i, j, length;
    scanf("%d", &N);
    for(int i = 0; i < N; i++) scanf("%d", origin + i);
    for(int i = 0; i < N; i++) scanf("%d", halfsort + i);

    /* if it is insertion sort, return sorted length if yes, zero otherwise */
    for(i = 0; i < N - 1 && halfsort[i] <= halfsort[i + 1]; i++) ;
    for(length = ++i; i < N && halfsort[i] == origin[i]; i++) ;
    length = i == N ? length + 1 : 0;

    if(length)                  /* insertion sort */
    {
        puts("Insertion Sort");
        qsort(origin, length, sizeof(int), comp);
    }
    else                        /* merge sort, operate on the original array */
    {
        puts("Merge Sort");
        for(length = 1, i = 0; i < N && length <= N; length *= 2)
        {
            /* i == N means identical, also breaks the outer 'for' loop */
            for(i = 0; i < N && origin[i] == halfsort[i]; i++) ;
            for(j = 0; j < N / length; j++)
                qsort(origin + j * length, length, sizeof(int), comp);
            qsort(origin + j * length, N % length, sizeof(int), comp);
        }
    }

    for(int i = 0; i < N; i++)
        printf("%d%c", origin[i], i == N - 1 ? '\n' : ' ');

    return 0;
}