圆方树做法另一疑问

#include <algorithm>
#include <cstdio>
#include <cassert>
using namespace std;
typedef long long ll;
const int maxn = 5e4 + 5, maxm = 6e4 + 5, inf = 0x3f3f3f3f;
struct Edge {
	int to, next;
} edge[maxm << 1];

int head[maxn], len;
void insert(const int &u, const int &v) {
	edge[++len] = {v, head[u]}, head[u] = len;
}

int dfn[maxn], low[maxn], fa[maxn], Tim, f[maxn][2], n, m;
void DP(const int &u, const int &v) {
	int g0, g1, f0 = 0, f1 = 0;
//
	for (int x = v; x != u; x = fa[x]) {
		g0 = f0 + f[x][0], g1 = f1 + f[x][1];
		f0 = max(g0, g1), f1 = g0;
	}
//
	f[u][0] += f0;
	f0 = 0, f1 = -inf;
	for (int x = v; x != u; x = fa[x]) {
		g0 = f0 + f[x][0], g1 = f1 + f[x][1];
		f0 = max(g0, g1), f1 = g0;
	}
	f[u][1] += f1;
}

void Tarjan(const int &u) {
	dfn[u] = low[u] = ++Tim, f[u][1] = 1;
	for (int i = head[u], v; i; i = edge[i].next) {
		if ((v = edge[i].to) == fa[u]) continue;
		if (!dfn[v = edge[i].to]) fa[v] = u, Tarjan(v), low[u] = min(low[u], low[v]);
		else low[u] = min(low[u], dfn[v]);
		if (low[v] > dfn[u]) f[u][0] += max(f[v][0], f[v][1]), f[u][1] += f[v][0];
	}
	for (int i = head[u], v; i; i = edge[i].next) {
		v = edge[i].to;
		if (fa[v] != u && dfn[v] > dfn[u])
			DP(u, v);
	}
}

int main() {
	scanf("%d%d", &n, &m);
	for (int u, v; m; --m) scanf("%d%d", &u, &v), insert(u, v), insert(v, u);
	Tarjan(1), printf("%d", max(f[1][0], f[1][1]));
	return 0;
}