#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
using namespace std;
bool lesslength(const string& s1, const string& s2)
{
    return s1.length()<s2.length();
}
int main()
{
    string s;
    vector<string>a;
    while((cin>>s))

    {

        a.push_back(s);

    }
    stable_sort(a.begin(),a.end(),lesslength);
    vector<string>::iterator pos;
    for ( pos = a.begin(); pos != a.end(); ++pos)
    {
        cout<<*pos<<endl;
    }
    return 0;
}

I've WA 2.

It's not just about length