c编写Iterative Mergesort

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本文介绍了如何不使用递归的方式实现归并排序。通过从长度为1的N个已排序子列表开始，每次合并相邻的子列表，直到得到一个完整的有序列表。关键在于实现合并子列表的函数。

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Iterative Mergesort (9分)

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 list into 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

主要考虑好特殊情况即可

写了两个版本。。第一个比较复杂

version1：

void merge_pass( ElementType list[], ElementType sorted[], int N, int length )
{
	int i=0,j=0;
	if(length*2<N){
		for(i=0;i+2*length-1<N;i=i+2*length){
			int count1=0,count2=0;
			while(count1<length&&count2<length){
				if(list[i+count1]<list[i+length+count2]){
					sorted[j++]=list[i+count1];
					count1++;
				}
				else{
					sorted[j++]=list[i+length+count2];
					count2++;
				}
			}
			while(count1<length){
				sorted[j++]=list[i+count1];
				count1++;
			}
			while(count2<length){
				sorted[j++]=list[i+length+count2];
				count2++;
			}
		}
		if(i+length>=N){
			while(i<N)
				sorted[j++]=list[i++];}
		else{
			int count1=0,count2=0;
			while(count1<length&&i+length+count2<N){
				if(list[i+count1]<list[i+length+count2]){
					sorted[j++]=list[i+count1];
					count1++;
				}
				else{
					sorted[j++]=list[i+length+count2];
					count2++;
				}
			}
			while(count1<length){
				sorted[j++]=list[i+count1];
				count1++;
			}
			while(i+length+count2<N){
				sorted[j++]=list[i+length+count2];
				count2++;
			}
		}
	}
	else {
		int p=0,q=length;
		while(p<length&&q<N){
			if(list[p]<list[q]){
				sorted[j++]=list[p++];
			}
			else{
				sorted[j++]=list[q++];
			}
		}
		while(p<length){
			sorted[j++]=list[p++];
		}
		while(q<N){
			sorted[j++]=list[q++];
		}
	}
	
}

version2：

void merge_pass( ElementType list[], ElementType sorted[], int N, int length ){
    int i, j, k, p = 0;
    
    for(i = 0; i < N; i += length * 2){
        if(i + length * 2 < N){
            j = i;
            k = j + length;
            while(j < i + length&&k < i + 2 * length){
                if(list[j] > list[k]){
                    sorted[p++] = list[k++];
                }
                else{
                    sorted[p++] = list[j++];
                }
            }
            while(j < i + length){
                sorted[p++] = list[j++];
            }
            while(k < i + 2 * length){
                sorted[p++] = list[k++];
            }
        }
        else if(i+length<N){
            j = i;
            k = j + length;
            while(j < i + length&&k < N){
                if(list[j] > list[k]){
                    sorted[p++] = list[k++];
                }
                else{
                    sorted[p++] = list[j++];
                }
            }
            while(j < i + length){
                sorted[p++] = list[j++];
            }
            while(k < N){
                sorted[p++] = list[k++];
            }
        }
        else{
            j = i;
            while(j < N){
                sorted[p++] = list[j++];
            }
        }
    }
}