mirror of
https://git.sb/baoshuo/OI-codes.git
synced 2024-12-25 18:52:06 +00:00
120 lines
1.8 KiB
C++
120 lines
1.8 KiB
C++
#include <iostream>
|
|
#include <queue>
|
|
#include <stack>
|
|
#include <unordered_set>
|
|
#include <utility>
|
|
#include <vector>
|
|
|
|
using std::cin;
|
|
using std::cout;
|
|
const char endl = '\n';
|
|
|
|
const int N = 1005;
|
|
|
|
int n, m, color[N], ans;
|
|
int cnt, id[N];
|
|
bool vis[N];
|
|
std::vector<int> g[N];
|
|
std::unordered_set<int> p[N];
|
|
|
|
bool dfs1(int u, int c) {
|
|
color[u] = c;
|
|
|
|
for (int v : g[u]) {
|
|
if (color[v]) {
|
|
if (color[v] == c) return false;
|
|
} else if (!dfs1(v, 3 - c)) {
|
|
return false;
|
|
}
|
|
}
|
|
|
|
return true;
|
|
}
|
|
|
|
bool check() {
|
|
for (int i = 1; i <= n; i++) {
|
|
if (!color[i]) {
|
|
if (!dfs1(i, 1)) return false;
|
|
}
|
|
}
|
|
|
|
return true;
|
|
}
|
|
|
|
void dfs2(int u, int x) {
|
|
id[u] = x;
|
|
p[x].insert(u);
|
|
|
|
for (int v : g[u]) {
|
|
if (!id[v]) dfs2(v, x);
|
|
}
|
|
}
|
|
|
|
int bfs(int x) {
|
|
std::fill_n(vis, N, false);
|
|
|
|
int res = 0;
|
|
|
|
std::queue<std::pair<int, int>> q;
|
|
|
|
q.emplace(x, 0);
|
|
vis[x] = true;
|
|
|
|
while (!q.empty()) {
|
|
auto e = q.front();
|
|
q.pop();
|
|
|
|
int u = e.first,
|
|
w = e.second;
|
|
|
|
res = std::max(res, w);
|
|
|
|
for (int v : g[u]) {
|
|
if (vis[v]) continue;
|
|
vis[v] = true;
|
|
|
|
q.emplace(v, w + 1);
|
|
}
|
|
}
|
|
|
|
return res;
|
|
}
|
|
|
|
int main() {
|
|
std::ios::sync_with_stdio(false);
|
|
cin.tie(nullptr);
|
|
|
|
cin >> n >> m;
|
|
|
|
for (int i = 1, u, v; i <= m; i++) {
|
|
cin >> u >> v;
|
|
|
|
g[u].emplace_back(v);
|
|
g[v].emplace_back(u);
|
|
}
|
|
|
|
if (!check()) {
|
|
cout << -1 << endl;
|
|
|
|
exit(0);
|
|
}
|
|
|
|
for (int i = 1; i <= n; i++) {
|
|
if (!id[i]) dfs2(i, ++cnt);
|
|
}
|
|
|
|
for (int i = 1; i <= cnt; i++) {
|
|
int res = 0;
|
|
|
|
for (const int &x : p[i]) {
|
|
res = std::max(res, bfs(x));
|
|
}
|
|
|
|
ans += res;
|
|
}
|
|
|
|
cout << ans << endl;
|
|
|
|
return 0;
|
|
}
|