foo.cc: In function 'int main()':
foo.cc:9:15: warning: 'n' is used uninitialized in this function [-Wuninitialized]
  for(int x=0;x<n;x++)cin>>P[n];
              ~^~
foo.cc:13:3: warning: 'w' may be used uninitialized in this function [-Wmaybe-uninitialized]
   if(P[left]+P[right]>w)right--,pair++;
   ^~