How can find difference in two codes?



----------------------1-------------------------

void addVertex(int & a)
{
    bool find = false;
    int len = v.size();
    int k = 0;
    for(int i = 0; i < len; i++)
    {
        if(v[i] == a)
        {
            a = i;
            k++;
            find = true;
        }
    }
    if(k > 1)
    {
        //throw 42;
    }
    if(!find)
    {
        v.push_back(a);
        a = v.size() - 1;
        part[a] = partNumber++;
    }
}



------------------------------2----------------------

void addVertex(int & a)
{
    int len = v.size();
    for(int i = 0; i < len; i++)
    {
        if(v[i] == a)
        {
            a = i;
            return;
        }
    }
    v.push_back(a);
    a = v.size() - 1;
    part[a] = partNumber++;
}

If I use second function I get AC, but if I use first function I get WA 1. If in first add string "throw 42" then I get Crash 1.

I use vector "v" only in this function.