tarjan求割点 为什么son++不能在if(!dfn[v])外

#include<bits/stdc++.h>
using namespace std;
int n,m,ut,vt,wt,root,dfn[100010],low[100010],cnt;
vector<int> g[100010];
bool iscut[100010];
vector<int> cut;
void tarjan(int u,int fa) {
	int son = 0;
	dfn[u] = low[u] = ++cnt;
	for (int i = 0;i < g[u].size();i ++) {
		int v = g[u][i];
		if (!dfn[v]) {
			++ son;
			tarjan(v,u);
			low[u] = min(low[u],low[v]);
			if (low[v] >= dfn[u] && u != root) iscut[u] = true;//////
		} else if (v != fa) low[u] = min(low[u],dfn[v]);//
	}
	if (u == root && son >= 2) iscut[u] = true;//////
	return;
}
int main() {
	scanf("%d%d",&n,&m);
	for (int i = 1;i <= m;i ++) {
		scanf("%d%d",&ut,&vt);
		g[ut].push_back(vt);
		g[vt].push_back(ut);
	}
	for (int i = 1;i <= n;i ++) {
//		memset(dfn,0,sizeof(dfn));
//		memset(low,0,sizeof(low));
		root = i;
		if (!dfn[i]) tarjan(i,0);
	}
	for (int i = 1;i <= n;i ++)
		if (iscut[i]) cut.push_back(i);
	printf("%d\n",cut.size());
	for (int i = 0;i < cut.size();i ++)
		printf("%d ",cut[i]);
	printf("\n");
	return 0;
}