zzu--2014年11月16日月賽 E題

zzu--2014年11月16日月賽 E題

Description

A permutation of length n is an integer sequence such that each integer from 0 to (n - 1) appears exactly once in it. For example, sequence [0, 2, 1] is a permutation of length 3 while both [0, 2, 2] and [1, 2, 3] are not.

A fixed point of a function is a point that is mapped to itself by the function. A permutation can be regarded as a bijective function. We'll get a definition of a fixed point in a permutation. An integer i is a fixed point of permutation a[0], a[1], ..., a[n-1] if and only if a[i]=i . For example, permutation [0, 2, 1] has 1 fixed point and permutation [0, 1, 2] has 3 fixed points.

You are given permutation a. You are allowed to swap two elements of the permutation at most once. Your task is to maximize the number of fixed points in the resulting permutation. Note that you are allowed to make at most one swap operation.

Input

The first line contains a single integer n (1 <= n <= 10^5). The second line contains n integers a[0], a[1], ..., a[n - 1] ---- the given permutation.

Output

Print a single integer — the maximum possible number of fixed points in the permutation after at most one swap operation.

Sample Input

Sample Output

HINT

題意：找出最多交換兩個數的位置的序列中a [ i ] = i 的數的個數！

思路：因為最多就交換一次，ans最多也就多2， 然而只要滿足了 i = a[ a[ i ] ] 就可以多2，否則多1或不變；

具體看代碼（水水）：

#include 
#include 
#include 
using namespace std;

int fun(int a[], int b[], int n)
{
	int flag=0;
	for(int i=0; i < n; i++)
	{
		if(b[i] == 0)
		{
			if(i == a[a[i]]) return 2;
			flag++;
		}
	}
	if(flag)return 1;
	else return 0;
}

int main()
{
	int n;
	int a[100010], is[100010];
	while(scanf("%d", &n) != EOF)
	{
		int ans=0;
		memset(is, 0, sizeof(is));
		memset(a, 0, sizeof(a));
		for(int i=0; i