Visual 3-way归并排序stackoverflow错误

嗨，伙计们，谁能告诉我我的3-way合并排序代码有什么问题?我写的代码只能排序4个数字，如果你给它超过4个数字(通过改变大小)，它最终与堆栈溢出错误，这是代码:

#include "stdafx.h"
#include <iostream>
#include <vector>
using namespace std;
const int size=4;
vector <int> s(size);
void merge(int,int,int);
void mergesort(int,int);
int main(){
    for(int i=0;i<size;i++){
        cout<<"enter number "<<i+1<<":";
        cin>>s.at(i);
    }
    system("CLS");
    cout<<"here are the unsorted numbers:n";//prints the input values so U can see'em
    for(int j=0;j<size;j++)
        cout<<s.at(j)<<".";
    mergesort(0,size-1);//calls mergesort
    cout<<"nhere are the sorted numbers:n";
    for(int j=0;j<size;j++)
        cout<<s.at(j)<<".";
    cin.get();
    cin.get();
    return 0;
}
void merge(int low,int one_third,int high){
    int i=low;
    int j=one_third+1;
    int k=0;
    int length=(high-low)+1;
    vector <int> u(length,0);
    if(k<length){
        while((i<=one_third)&&(j<=high)){
            if(s.at(i)<=s.at(j)){
                u.at(k)=s.at(i);
                i++;
                k++;
            }//end for
            else{
                u.at(k)=s.at(j);
                j++;
                k++;
            }//end elseif
        }//end while
        if(j>high)
            while(i<=one_third)
            {
                u.at(k)=s.at(i);
                i++;
                k++;
            }
        if(i>one_third)
            while(j<=high)
            {
                u.at(k)=s.at(j);
                j++;
                k++;
            }
        for(int n=low;n<k;n++)
            s.at(n)=u.at(n);
    }
}//end if
void mergesort(int low,int high){
    if(low<high){
        int one_third=(high-low)/3;//division,it's 3-way mergesort so obviously it's divided by 3
        int two_third=2*one_third;
        mergesort(low,one_third);
        mergesort(one_third+1,two_third);
        mergesort(two_third+1,high);
        merge(low,one_third,two_third);
        merge(low,two_third,high);
    }//end if
}