被 Hack 力！

WA on Test #11, #13，求大佬看看哪错了。

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int N = 1e3 + 5;
struct node{
	int v, nxt;
}e[N * N * 2];
int n, a[N];
int stk1[N], stk2[N], tot1, tot2;
int rm[N], col[N];
int head[N], cnt;
int b[N];

void ad(int u, int v){
	e[++cnt].v = v, e[cnt].nxt = head[u];
	head[u] = cnt;
}

bool dfs(int u){
	for(int i = head[u]; i; i = e[i].nxt){
		int v = e[i].v;
		if(col[v] == col[u])	return false;
		if(col[v] != -1)	continue;
		col[v] = !col[u];
		dfs(v);
	}
	return true;
}

int main(){
	ios::sync_with_stdio(0);
	cin.tie(0);
	cin>>n;
	for(int i = 1;i <= n;i++){
		cin>>a[i];
		b[i] = a[i];
	}
	sort(b + 1, b + n + 1);
	rm[n] = a[n];
	for(int i = n-1;i;i--){
		rm[i] = min(rm[i+1], a[i]);
	}
	for(int i = 1;i <= n;i++){
		for(int j = i + 1;j + 1 <= n;j++){
			if(a[i] < a[j] && a[i] > rm[j + 1]){
//				cout<<i<<" "<<j<<"\n";
				ad(i, j);
				ad(j, i);
			}
		}
	}
	
	memset(col, -1, sizeof col);
	bool f = 1;
	for(int i = 1;i <= n;i++){
		if(col[i] == -1){
			col[i] = 1;
			f = f && dfs(i);
		}
	}
	if(!f){
		cout<<"0\n";
		return 0;
	}
	cout<<"a ";
	stk1[++tot1] = 1;
	int now = 1;
	int scnt = 1;
	while(tot1 || tot2){
		now++;
		while(tot1 >= 1 && a[stk1[tot1]] == b[scnt]){
			scnt++;
			cout<<"b ";
			tot1--;
		}
		while(tot2 >= 1 && a[stk2[tot2]] == b[scnt]){
			scnt++;
			cout<<"d ";
			tot2--;
		}
		if(now > n)	continue;
		if(tot1 == 0){
			stk1[++tot1] = now;
			cout<<"a ";
		}
		else if(col[stk1[tot1]] == col[now]){
			stk1[++tot1] = now;
			cout<<"a ";
		}
		else{
			stk2[++tot2] = now;
			cout<<"c ";
		}
//		for(int i = 1;i <= tot1;i++)	cout<<stk1[i]<<" \n"[i == tot1];
//		for(int i = 1;i <= tot2;i++)	cout<<stk2[i]<<" \n"[i == tot2];
	}
	return 0;
}