算法 1.2
void select_sort(int a[], int n) {
      // 將a中整數序列重新排列成自小至大有序的整數序列。
      for ( i = 0;  i< n-1;  ++i ) {
        j = i; 
        for ( k = i+1;  k < n;  ++k )
          if (a[k] < a[j] )  j = k;
        if ( j != i ) { w = a[j]; a[j] = a[i]; a[i] = w;}
} // select_sort