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3275：Ranking the Cows

题目描述

Each of Farmer John's N cows (1 ≤ N ≤ 1,000) produces milk at a different positive rate, and FJ would like to order his cows according to these rates from the fastest milk producer to the slowest.

FJ has already compared the milk output rate for M (1 ≤ M ≤ 10,000) pairs of cows. He wants to make a list of C additional pairs of cows such that, if he now compares those C pairs, he will definitely be able to deduce the correct ordering of all N cows. Please help him determine the minimum value of C for which such a list is possible.

输入解释
Line 1: Two space-separated integers: N and M
Lines 2..M+1: Two space-separated integers, respectively: X and Y. Both X and Y are in the range 1...N and describe a comparison where cow X was ranked higher than cow Y.
输出解释
Line 1: A single integer that is the minimum value of C.
输入样例 
5 5
2 1
1 5
2 3
1 4
3 4
输出样例 
3
提示
From the information in the 5 test results, Farmer John knows that since cow 2 > cow 1 > cow 5 and cow 2 > cow 3 > cow 4, cow 2 has the highest rank. However, he needs to know whether cow 1 > cow 3 to determine the cow with the second highest rank. Also, he will need one more question to determine the ordering between cow 4 and cow 5. After that, he will need to know if cow 5 > cow 3 if cow 1 has higher rank than cow 3. He will have to ask three questions in order to be sure he has the rankings: "Is cow 1 > cow 3? Is cow 4 > cow 5? Is cow 5 > cow 3?"
该题目是Virtual Judge题目，来自 北京大学POJ
题目来源 USACO 2007 March Gold
源链接： POJ-3275
最后修改于 2020-10-29T06:57:31+00:00 由爬虫自动更新
共提交 0 次
通过率 --%
时间上限 内存上限
2000 65536