Compilare: main.cpp: In function 'int main()': main.cpp:94:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int j = 0; j < X[i].size(); ++j) { ~~^~~~~~~~~~~~~ main.cpp:98:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (j < X[i].size() - 1) up = X[i][j + 1]; ~~^~~~~~~~~~~~~~~~~