int top(si &s1, si& s2){ if(!s2.empty()) return s2.top(); if(s1.empty()) return -1; while(!s1.empty()){ s2.push(s1.top()); s1.pop(); } return s2.top(); }
void pop(si &s1, si &s2){ if(!s2.empty()){ s2.pop(); return; } if(s1.empty()) return; while(true){ int i=s1.top(); s1.pop(); if(s1.empty()) return; s2.push(i); } }