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<b>例5.9 </b> 考虑表达式&nbsp;<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;       
a＋a*(b-c)+(b-c)*d&nbsp;<br>
利用上述方法构造出的相应的有向非循环图如下：
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图中叶结点a有两个父结点，因为a是两个子表达式a和a*(b-c)的公共子表达式。同样，b-c是两个子表达式a*（b－c）和（b－c）*d的公共子表达式，其相应结点也有两个父结点。上述有向非循环图是自底向上构造的，需要进行下面一系列的函数调用：
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(1) p<span class="down"><sub>1</sub></span>:=mkleaf(<b>id</b>,a);<br> 
&nbsp;&nbsp;&nbsp; (2) p<span class="down"><sub>2</sub></span>:=mkleaf(<b>id</b>,a);<br> 
&nbsp;&nbsp;&nbsp; (3) p<span class="down"><sub>3</sub></span>:=mkleaf(<b>id</b>,b);<br> 
&nbsp;&nbsp;&nbsp; (4) p<span class="down"><sub>4</sub></span>:=mkleaf(<b>id</b>,c);<br> 
&nbsp;&nbsp;&nbsp; (5) p<span class="down"><sub>5</sub></span>:=mknode('-',p<span class="down"><sub>3</sub></span>,p<span class="down"><sub>4</sub></span>);<br> 
&nbsp;&nbsp;&nbsp; (6) p<span class="down"><sub>6</sub></span>:=mknode('*',p<span class="down"><sub>2</sub></span>,p<span class="down"><sub>5</sub></span>);<br> 
&nbsp;&nbsp;&nbsp; (7) p<span class="down"><sub>7</sub></span>:=mknode('+',p<span class="down"><sub>1</sub></span>,p<span class="down"><sub>6</sub></span>);<br> 
&nbsp;&nbsp;&nbsp; (8) p<span class="down"><sub>8</sub></span>:=mkleaf(<b>id</b>,b);<br> 
&nbsp;&nbsp;&nbsp; (9) p<span class="down"><sub>9</sub></span>:=mkleaf(<b>id</b>,c);<br> 
&nbsp;&nbsp;&nbsp;&nbsp;(10) p<span class="down"><sub>10</sub></span>:=mknode('-',p<span class="down"><sub>8</sub></span>,p<span class="down"><sub>9</sub></span>);<br> 
&nbsp;&nbsp;&nbsp;&nbsp;(11) p<span class="down"><sub>11</sub></span>:=mkleaf(<b>id</b>,d);<br> 
&nbsp;&nbsp;&nbsp;&nbsp;(12) p<span class="down"><sub>12</sub></span>:=mknode('*',p<span class="down"><sub>10</sub></span>,p<span class="down"><sub>11</sub></span>);<br> 
&nbsp;&nbsp;&nbsp;&nbsp;(13) p<span class="down"><sub>13</sub></span>:=mknode('+',p<span class="down"><sub>7</sub></span>,p<span class="down"><sub>12</sub></span>);<br> 
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<p>仅当需要时由mknode和mkleaf建立新的结点，如果与新的结点相同的结点已经存在，那么返回一个指针，此指针指向原有的结点。上述序列中的a,b,c和d都表示指针，并分别指向标识符a,b,c和d的符号表的表项。 
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<p>当重复调用第2行上的mkleaf(<b>id</b>,a)时，因为相同的调用已用第1行上的mkleaf(<b>id</b>,a)进行过了，因而无需建立新的叶结点只需返回指针p<span class="down"><sub>1</sub></span>，所以p<span class="down"><sub>l</sub></span>＝p<span class="down"><sub>2</sub></span>。与此类似，在第8行和第9行上的调用将分别返回p<span class="down"><sub>3</sub></span>和p<span class="down"><sub>4</sub></span>。因此第10行和第5行是同样的，它们所要建立的结点是相同的。 
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