/in/foo.cc: In function 'int main()':
/in/foo.cc:22:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     if(w.size()<k-1)
        ~~~~~~~~^~~~
/in/foo.cc:36:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     if(w.size()<k-1)
        ~~~~~~~~^~~~
/in/foo.cc:40:16: warning: suggest parentheses around comparison in operand of '^' [-Wparentheses]
      if(w.top()>x^u)
         ~~~~~~~^~