合并排序无法正常工作？

Question

void mergeSubArr(int arr[], int lb, int mid, int ub){
    int temp[size], i = lb, j = mid, k = 0;
    while(i<mid && j<=ub){
        if(arr[i] <= arr[j])
            temp[k] = arr[i++];
        else
            temp[k] = arr[j++];
        k++;
    }
    while(i<mid)
        temp[k++] = arr[i++];
    while(j<=ub)
        temp[k++] = arr[j++];

    for(k=0;k<size;k++)
        arr[k] = temp[k];

}

void mergeArr(int num[], int lb, int ub){
    int mid;
    if((ub-lb)>1){
        mid = (lb+ub)/2;
        mergeArr(num, lb, mid);
        mergeArr(num, mid+1, ub);
        mergeSubArr(num, lb, mid+1, ub);
    }
}

when calling the function mergeArr the output is outputting some other elements which are not initially present in the array? I think there is something wrong with the mergeSubArr function please help me out here in finding a solution.

Answer 1

There are basically two things wrong with your algorithm:

You fill the array temp from index 0 on. Later, you need to copy it to arr , but starting at the lower bound. You also shouldn't copy size elements, because you can't be sure that the original array even has size elements. (Thanks, BLUEPIXY, for spotting this one.) You should probably also make sure that size is big enough to hold all elements.

Your upper bound is inclusive. That means that if (ub - lb > 1) catches the case where there are more than two elements. You still have to treat the case where there are two elements, which may be in the wrong order, either by making the above condition ub - lb > 0 or by having an else clause where you swap the elements if they are in the wrong order.

So make these changes:

int temp[size], i = lb, j = mid, k = lb;

Start the temp array from the lower bound.

for(k = lb; k <= ub; k++)
    arr[k] = temp[k];

Copy only the subarray in question.

if (ub - lb > 0) ...

Catch the case where there are two elements, too.

On a personal note, I find the mergesort algorithm much easier when it passes subarrays via pointer arithmetic and length. It forgoes much fiddling with offsets.

Answer 2

All are okk. Instead if((ub-lb)>1) Just replace

if((ub-lb)>0)

OR

if(ub > lb)