IwfWcf's OI Log: ZJU/ZOJ 2284 Inversion Number 解题报告

2008年10月2日星期四

ZJU/ZOJ 2284 Inversion Number 解题报告

题目大意是求n的全排列中逆序对数为k的排列数。

n的全排列可以看作是将n插入n-1的全排列中生成的，而对于n-1的全排列来说，n有n个插入位置，将n插入第i个位置可以增加(n-i)对逆序对。所以可以得到递推式f[i,j]=∑f[i-1,j-k](0<=k<=i-1)，边界条件是f[1,0]=1。

1653710 2008-10-02 11:39:31 Accepted 2284 FPC 0 144 IwfWcf

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