2.5 Divide Conquer

• binary search:
  1. only compare once, just
  2. avoid s+e overflow.
  3. s will catch up e in the end, then break while loop
  4. check a[m] == k outside of while
  while(s<e) { 
      //m=(s+e)/2; 
      m = s/2+e/2 + s&e&1; 
      if(a[m]<k) 
          s = m+1; 
      else 
         e = m; 
  } 
  if(a[m] == k) 
         return ture;