NOIP2015提高组day1 —— 信息传递（message）

#include <fstream>
#include <algorithm>
using namespace std;
ifstream fin("message.in");
ofstream fout("message.out");

const int MAX_n = 200000;
int ancestor[MAX_n], transfer[MAX_n];

int union_find(int root) {
    if (ancestor[root] == root)
        return root;
    else
        return ancestor[root] = union_find(ancestor[root]);
}

int depth(int initial, int root) {
    int sum = 1;
    for (int i = initial; i != root; i = transfer[i])
        sum++;
    return sum;
}

int main() {    
    int n;
    fin >> n;
    for (int i = 0; i < n; i++)
        ancestor[i] = transfer[i] = i;
    int ans = n;
    for (int i = 0; i < n; i++) {
        fin >> transfer[i];
        int roots = union_find(transfer[i]);
        if (i != roots)
            ancestor[i] = roots;
        else
            ans = min(ans, depth(transfer[i], i));
    }
    fout << ans << endl;
    return 0;
}