算法 5.2
void Concat_Sq( char S1[ ], char S2[ ], char T[ ]) {
       // 用T返回由S1和S2聯接而成的新串
       j=0;  k=0; 
       while ( S1[j] != '\0' )  T[k++] = S1[j++];  //復制串S1
       j = 0;
       while ( S2[j] != '\0' )  T[k++] = S2[j++];  //接著復制串S2 
       T[k] = '\0';                                //置結果串T的結束標志
} // Concat