PTA Iterative Mergesort

How would you implement mergesort without using recursion?

The idea of iterative mergesort is to start from N sorted sublists of length 1, and each time to merge a pair of adjacent sublists until one sorted list is obtained. You are supposed to implement the key function of merging.

Format of functions:

void merge_pass( ElementType list[], ElementType sorted[], int N, int length );

The function merge_pass performs one pass of the merge sort that merges adjacent pairs of sublists from listinto sorted. N is the number of elements in the list and length is the length of the sublists.

Sample program of judge:

#include <stdio.h>

#define ElementType int
#define MAXN 100

void merge_pass( ElementType list[], ElementType sorted[], int N, int length );

void output( ElementType list[], int N )
{
    int i;
    for (i=0; i<N; i++) printf("%d ", list[i]);
    printf("\n");
}

void  merge_sort( ElementType list[],  int N )
{
    ElementType extra[MAXN];  /* the extra space required */
    int  length = 1;  /* current length of sublist being merged */
    while( length < N ) {
        merge_pass( list, extra, N, length ); /* merge list into extra */
        output( extra, N );
        length *= 2;
        merge_pass( extra, list, N, length ); /* merge extra back to list */
        output( list, N );
        length *= 2;
    }
} 

int main()
{
    int N, i;
    ElementType A[MAXN];

    scanf("%d", &N);
    for (i=0; i<N; i++) scanf("%d", &A[i]);
    merge_sort(A, N);
    output(A, N);

    return 0;
}

/* Your function will be put here */

Sample Input:

10
8 7 9 2 3 5 1 6 4 0

Sample Output:

7 8 2 9 3 5 1 6 0 4
2 7 8 9 1 3 5 6 0 4
1 2 3 5 6 7 8 9 0 4
0 1 2 3 4 5 6 7 8 9
0 1 2 3 4 5 6 7 8 9

解答

这题有一个要注意的地方，就是每次归并最后剩下的数列长记为x，如果x <= length的话，就不需要归并了，直接赋值，如果x > length的话，就是一个长为length的子列和一个长为x - length的子列归并。

void merge_pass( ElementType list[], ElementType sorted[], int N, int length ){
    ;

    ; i < N; i += length * ){
         < N){
            j = i;
            k = j + length;
             * length){
                if(list[j] > list[k]){
                    sorted[index++] = list[k++];
                }
                else{
                    sorted[index++] = list[j++];
                }
            }
            while(j < i + length){
                sorted[index++] = list[j++];
            }
             * length){
                sorted[index++] = list[k++];
            }
        }
        else if(N - i > length){
            j = i;
            k = j + length;
            while(j < i + length&&k < N){
                if(list[j] > list[k]){
                    sorted[index++] = list[k++];
                }
                else{
                    sorted[index++] = list[j++];
                }
            }
            while(j < i + length){
                sorted[index++] = list[j++];
            }
            while(k < N){
                sorted[index++] = list[k++];
            }
        }
        else{
            j = i;
            while(j < N){
                sorted[index++] = list[j++];
            }
        }
    }
}