Question

我一直在进行合并排序，我已经为它编写了一些代码，但它不能按照给定的算法运行。我完全根据算法做了但似乎有些不对劲。请有人让我知道什么是错的。

#include<stdio.h>

int main(){
    int a[] = {5, 3, 2, 4, 9, 1, 11, 6, 10};
    mergeSort(a, 0, sizeof(a)/ sizeof(int));
    printArray(a);
}

void mergeSort(int a[], int low, int high){
    int med;
    if(high - low > 1){
        med = (high - low) / 2 + low;
        mergeSort(a, low, med);
        mergeSort(a, med+1, high);
        merge(a, low, med, high);
    }
}
void printArray(int a[]){
    int i;
    for(i = 0; i < 9; i++){
        printf("%d ", a[i]);
    }
    printf("\n");
}
void merge(int a[], int low, int med, int high){
    int i = 0, j = 0, k = low;
    int a1[9];
    int a2[9];
    for(i = 0; i < med; i++){
        a1[i] = a[i];
    }

    for(i = med; i < high; i++){
        a2[i] = a[i];
    }

    while(i < high - med && j < med - low){
        if(a1[i] <= a2[j]){
            a[k] = a1[i];
            i++;
        }else{
            a[k] = a2[j];
            j++;
        }
    }

    while(i < high - med){
        a[k] = a1[i];
        i++;
    }

    while(j < med - low){
        a[k] = a2[j];
        j++;
    }

}

算法：http://i.stack.imgur.com/f73Zi.jpg

Answer 1

我不知道你从哪里得到那个算法但是它的结尾（最后2个while循环）是不正确的。你也错误地复制了部分内容。我会遗漏那些没有变化且没必要的东西。

我的所有评论都指出了代码的更改位置。

void mergeSort(int a[], int low, int high){
    int med;
    if(high - low > 1){
        med = (high - low) / 2 + low;
        mergeSort(a, low, med);
        mergeSort(a, med, high); //you copied this down wrong. med NOT med + 1
        merge(a, low, med, high);
    }
}
void merge(int a[], int low, int med, int high){
    int i = 0;
    int j = 0;
    int k = low;
    int a1[9];
    int a2[9];
    for(i = 0; i < med; i++){
        //algorithm states to copy by doing: a1[0 -> med] = a[low -> med - 1]
        //we're copying the subarray 'a' from 'low' to 'med - 1' into
        //'a1' from 0 to 'med'
        a1[i] = a[i + low];
    }

    for(i = med; i < high; i++){
        //algorithm states to copy by doing: a2[0 -> med] = a[med -> high - 1]
        a2[i - med] = a[i];
    }

    //don't forget to reset i to 0
    i = 0;

    while(i < med - low && j < high - med){
        if(a1[i] <= a2[j]){
            a[k] = a1[i];
            i++;
        }else{
            a[k] = a2[j];
            j++;
        }
        k++;
    }

    while(i < med - low){
        a[k] = a1[i];
        i++;
        k++;
        //algorithm didn't have k++ for some reason
    }

    while(j < high - med){
        //note it's a2[j] not a2[i]
        //algorithm was wrong here
        a[k] = a2[j];
        j++;
        k++;
        //didn't have k++ here either
    }
}

最后3个while循环可以通过数组索引中的后增量运算符缩短，因为我们知道我们的意图是在语句完成后递增它们（因此 post -increment）。 / p>

    while(i < med - low && j < high - med){
        if(a1[i] <= a2[j])
            a[k++] = a1[i++];
        else
            a[k++] = a2[j++];
    }

    while(i < med - low)
        a[k++] = a1[i++];

    while(j < high - med)
        a[k++] = a2[j++];

合并排序不起作用

1 个答案: