Compilare: main.cpp: In function 'int main()': main.cpp:24:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0; i<s.size(); i++) ~^~~~~~~~~ main.cpp:31:24: warning: operation on 'semn' may be undefined [-Wsequence-point] st[++semn]=-st[semn-1]; ^~~~~~ main.cpp:33:24: warning: operation on 'semn' may be undefined [-Wsequence-point] st[++semn]=st[semn-1]; ^~~~~~