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北京大学《ACM/ICPC大学生程序设计竞赛训练》暑期课面向全球招生!

可证.昨晚证过了.内详.

Posted by Assassin_cpy at 2008-12-26 12:38:27 on Problem 2631
In Reply To:在某个论坛看到有人提供的一个待验证的算法,刚试了一下,是正确的,方法如下 Posted by:czcomt at 2008-12-25 18:34:14
设最长链是MN->已知[1]
设由B开始DFS得到最长路为BC->已知[2]
结论[1] MN与AB有公共点.否则MN<AM+AN<=AM+AB=BM 与已知[1]矛盾
结论[2] B是最长链的一个端点.否则由结论[1] 设K是AB上距B最近且在MN上的点 则MN=MK+KN=MK+AN-AK<=MK+AB-AK=MK+BK=BM 当取等号时MB与MN等长 符合结论[2] 否则与已知[1]矛盾  [这里假定了A不在NK上.若A在NK上 只须将上面式子中MN交换位置即可 不影响结论]
结论[3] BC是一条最长链.否则由结论[2] 设经过B的最长链为BD 则BD>BC 与已知[2]矛盾
至此证毕

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