算法 2.10
void exchang1( SqList &A, int m, int n ) {
      // 本算法實現順序表中前 m 個元素和后 n 個元素的互換
      for (k = 1; k<=n; k++ ) {
         w = A.elem[m+k-1];
         for ( j=m+k-1; j>=k; j-- )  A.elem[j] = A.elem[j-1];
         A.elem[k-1] = w;
       } // for
} // exchang1