/in/foo.cc: In function 'int main()':
/in/foo.cc:9:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for(int i=1;i<st.length()-i;i++) swap(st[i],st[st.length()-i]);
                ^
/in/foo.cc:12:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<st.length()-1-i;i++) swap(st[i],st[st.length()-1-i]);
                  ^