O2 前

O2 后

我的程序 UB 了？

#include <bits/stdc++.h>
using namespace std;

int read(){
	int f = 1, x = 0;
	char c = getchar();
	while(!isdigit(c)){
		if(c == '-') f=-1;
		c=getchar();
	}
	while(isdigit(c)) {x=(x<<3)+(x<<1)+c-48, c=getchar();}
	return x * f;
}

struct node{
	int to, id;
	bool operator < (const node &a) const {
		return to < a.to;
	}
};
int n, m, u, v, cnt = 1;
int in[100010], out[100010];
int stk[100010], top;
int s, t;
bool vis[200010];
vector<node> g[200010];

void addedge(int uu, int vv){
	g[uu].push_back((node){vv, cnt++});
	in[vv]++, out[uu]++;
//	for(int i=1; i<=n; i++){
//		printf("%3d %3d\n", in[i], out[i]);
//	}
}

void dfs(int nw){
	for(int i=0; i<g[nw].size(); i++){
		if(vis[g[nw][i].id]) continue;
//		cout<<nw<<"-"<<g[nw][i].to<<endl;
		vis[g[nw][i].id] = true;
		dfs(g[nw][i].to);
	}
	stk[++top] = nw;
}

bool judge(){
	int c = 0;
	for(int i=1; i<=n; i++){
		if(in[i] != out[i]){
			c++;
			if(in[i] == out[i] - 1) s = i;
			if(in[i] == out[i] + 1) t = i;
			if(out[i]-in[i] > 1) return false;
		}
	}
	if(c != 0 && c != 2) return false;
	else if(c == 0) return s = t = 1, 1;
	if(!s || !t) return false;
	return true;
}

int main(){
	n = read(), m = read();
	for(int i=1; i<=m; i++){
		u = read(), v = read();
//		if(u == v) continue;
		addedge(u, v);
	}
	if(!judge()){
		puts("No");
		return 0;
	}
	for(int i=1; i<=n; i++){
		sort(g[i].begin(), g[i].end());
	}
	dfs(s);
	while(top){
		printf("%d ", stk[top--]);
	}
	return 0;
}