1、if(x<=0||y<=0)不满足,所以return 3*foo(x-6,y/2)即3*foo(20-6,13/2)-----3*foo(14,6);
2、再看foo(14,6), if(x<=0||y<=0)还不满足,return 3*3*foo(14-6,6/2)即3*3*foo(8,3);
3、再看foo(8,3), if(x<=0||y<=0)还不满足,return 3*3*3*foo(8-6,3/2)即3*3*3*foo(2,1);
4、再看foo(2,1), if(x<=0||y<=0)还不满足,return 3*3*3*3*foo(2-6,1/2)即3*3*3*3*foo(-4,0);
5、此时if(x<=0||y<=0)终于满足,即foo(-4,0)return 1,加上前面几步的迭代最终结果是3*3*3*3*1=81
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