不使用库函数求一个数的立方根

 1 double fun(double x) {
 2     if (x == 0) return 0;
 3     double low = 0;
 4     double top = x;
 5     if (x < 0) {
 6         low = x;
 7         top = 0;
 8     }
 9     while (low<top)
10     {
11         double mid = (low + top) / 2;
12         double tmp = mid*mid*mid;
13         if (tmp > x) {
14             top = mid;
15         }
16         else if (tmp < x) {
17             low = mid;
18         }
19         else {
20             return mid;
21         }
22     }
23 }