class Solution {

 public:

     int mySqrt(int x) {

         if(x ==  || x == ) return x;

         int beg = ;

         int end = x;

         long mid = ;   //这里用long，否则会溢出

         while(beg <= end){

             mid = beg + (end - beg)/;

             if(mid * mid < x){

                 beg = mid + ;

             }else if(mid * mid > x){

                 end = mid - ;

             }else{

                 return mid;

             }

         }

         return end;

     }

 };