mergesort algorithm in c++

问题

i have following code

#include <iostream>
using namespace std;
void merge(int c[],int a[],int n,int b[],int m){
         for (int i=0,j=0,k=0;k<n+m;k++){

             if (i==n) {  c[k]=b[j++]; continue;}
             if (j==m ){  c[k]=a[i++];continue;}

             c[k]=(a[i]<b[j])? a[i++]:b[j++];

         }



}
void mergesort(int a[],int b[],int l,int r){
if (l>r)  return ;
     int m=(l+r)/2;

     mergesort(b,a,l,m);
     mergesort(b,a,m+1,r);

 merge(a+l,b+l,m-l+1,b+m+1,r-m);

}
int main(){


    int a[]={12,4,2,5,3,6,7,8,10,11};
const   int n=sizeof(a)/sizeof(int);
int b[n];
mergesort(a,b,0,n-1);
    for (int i=0;i<n;i++){

        cout<<b[i]<< "  ";

    }




     return 0;
}

but here is such warning

1>------ Rebuild All started: Project: mergesort, Configuration: Debug Win32 ------
1>  mergesort.cpp
1>d:\c++_algorithms\mergesort\mergesort\mergesort.cpp(24): warning C4717: 'mergesort' : recursive on all control paths, function will cause runtime stack overflow
1>  mergesort.vcxproj -> D:\c++_algorithms\mergesort\Debug\mergesort.exe
========== Rebuild All: 1 succeeded, 0 failed, 0 skipped ==========

please help me to fix it

i have updated my code

回答1:

mergesort() needs to detect whether it's been asked to sort a list of length one, and if so, return immediately. Also, I could be wrong, but I think the call to merge() goes after the recursive calls to mergesort().

回答2:

Your recursive call should have a terminating condition so that the recursive calls would end at some point.
I think adding the following condition at the beginning of your mergesort function solves the problem:

if(l>r)
    return;

回答3:

You need to have a base case, which probably means that you should check whether the size of the array you are sorting is =1, and if it is, you return that particular element itself.

In code, this would mean checking something like

l==r

Also, logically, merge should be called after you have sorted the two sub-arrays.

回答4:

you need to specify a base case in your mergeSort function! and also, in the mergeSort algorithm, the call to merge comes after the recursive call to the left and right subarrays

来源：https://stackoverflow.com/questions/4199224/mergesort-algorithm-in-c

标签

c++

mergesort