Compilare: user.cpp: In function ‘int main(int, const char**)’: user.cpp:30:38: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for( int i = 0; i < str.size(); i ++ ) ^ user.cpp:33:39: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for( int l = 2; l <= str.size(); l ++ ) { ^ user.cpp:34:49: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for( int i = 0, j = l - 1; j < str.size(); i ++, j ++ ) { ^