Compilare: main.cpp: In function 'int main()': main.cpp:17:43: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while ( !st.empty() and st.size() >= k - ( n - i - 1 ) and st.top() < ch ) ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~ main.cpp:19:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if( st.size() < k) ~~~~~~~~~~^~~