C++ :

#include <iostream>
#include <algorithm>
#include <stack>
#include <string>
using namespace std;

const int N = 1007;
int a[N], c[N], Afterwards[N], n;
stack<int> s1, s2;
bool G[N][N];

bool Color(int x, int Colour)
{
	c[x] = Colour;
	for (int i = 1; i <= n; ++i)
		if (G[x][i])
			if (c[x] == c[i]) return false;
			else if (!c[i] && !Color(i, -Colour)) return false;
	return true;
}

int main()
{
	int i, j, k, Top = 1;
    string s = "";
	
	cin >> n;
	for (i = 1; i <= n; ++i) cin >> a[i];
	copy(a + 1, a + n + 1, Afterwards + 1);
	sort(Afterwards + 1, Afterwards + n + 1);
	
	for (i = 1; i <= n; ++i)
		for (j = i + 1; j <= n; ++j)
			if (a[i] < a[j])
				for (k = j + 1; k <= n; ++k)
					if (a[k] < a[i]) G[i][j] = G[j][i] = true;
	
	for (i = 1; i <= n; ++i)
		if (!c[i] && !Color(i, 1)) {cout << 0 << endl; return 0;}
	for (i = 1; i <= n; ++i)
		if (c[i] == 1)
		{
			s += "a "; s1.push(a[i]);
			while (!s1.empty() && s1.top() == Afterwards[Top]) {s += "b "; s1.pop(); ++Top;}
		}
		else {
			s += "c "; s2.push(a[i]);
			while (!s2.empty() && s2.top() == Afterwards[Top]) {s += "d "; s2.pop(); ++Top;}
		}
		while (!s1.empty() || !s2.empty())
			if (!s1.empty() && s1.top() == Afterwards[Top]) {s += "b "; s1.pop(); ++Top;}
			else if (!s2.empty() && s2.top() == Afterwards[Top]) {s += "d "; s2.pop(); ++Top;}
	cout << s.substr(0, s.size() - 1) << endl;
	
	return 0;
}

Pascal :

var
 n,i,j,st1,st2,next_num,min,t:longint;
 q1,color,s1,s2:array[0..1500] of longint;
 map:array[1..1500,1..1500] of boolean;
 vis:array[1..1500] of boolean;
 ans:array[1..3000] of char;
procedure print;
begin
 writeln(0);
 halt;
end;
procedure dfs(x,nowcolor:longint);
var
 i:longint;
begin
 vis[x]:=true;
 color[x]:=nowcolor;
 for i:=1 to n do
  if map[x,i] then begin
   if not vis[i] then dfs(i,3-nowcolor)
   else if color[i]<>3-nowcolor then print;
  end;
end;
begin
 readln(n);
 for i:=1 to n do read(q1[i]);
 readln;
 min:=maxlongint;
 for i:=n downto 1 do begin
  for j:=1 to i-1 do if (q1[j]<q1[i]) and (min<q1[j]) then begin
   map[q1[i],q1[j]]:=true;
   map[q1[j],q1[i]]:=true;
  end;
  if q1[i]<min then min:=q1[i];
 end;
 fillchar(vis,sizeof(vis),0);
 for i:=1 to n do if not vis[q1[i]] then dfs(q1[i],1);
 i:=1;
 next_num:=1;
 t:=q1[i];
 st1:=0;
 st2:=0;
 j:=0;
 while (i<=n) or (next_num<=n) do begin
  if (i<=n) and (color[t]=1) and ((st1=0) or (t<s1[st1])) then begin
   inc(j);
   ans[j]:='a';
   inc(st1);
   s1[st1]:=t;
   inc(i);
   if i<=n then t:=q1[i] else t:=0;
   continue;
  end;
  if (st1>0) and (s1[st1]=next_num) then begin
   inc(j);
   ans[j]:='b';
   dec(st1);
   inc(next_num);
   continue;
  end;
  if (i<=n) and (color[t]=2) and ((st2=0) or (t<s2[st2])) then begin
   inc(j);
   ans[j]:='c';
   inc(st2);
   s2[st2]:=t;
   inc(i);
   if i<=n then t:=q1[i] else t:=0;
   continue;
  end;
  if (st2>0) and (s2[st2]=next_num) then begin
   inc(j);
   ans[j]:='d';
   dec(st2);
   inc(next_num);
   continue;
  end;
 end;
 for i:=1 to j-1 do write(ans[i],' ');
 writeln(ans[j]);
end.

Java :

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
import java.util.Stack;

public class Main {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int size = sc.nextInt();
        List<Integer> stackList = new ArrayList<>();
        for (int i = 0; i < size; i++) {
            stackList.add(sc.nextInt());
        }

        Stack<Integer> stack1 = new Stack<>();
        Stack<Integer> stack2 = new Stack<>();
        List<Integer> result = new Stack<>();
        List<String> operation = new ArrayList<>();
        int i = 0;

        for (int j = i; j < size; j++) {
//            int k = i;
            // 一直丢进栈直到找出最小
            while (i<size&&stackList.get(i) != result.size()+1) {
                // 丢到1中有个必要条件就是2中的数要大于1中两个数之间的最小数
                boolean canPush1 = stack2.empty()||stackList.get(i)+1<stack2.get(stack2.size()-1)||stackList.get(i)+1==stack1.get(stack1.size()-1);
                if (stack1.size() == 0 || (stack1.get(stack1.size() - 1) >= stackList.get(i)&&canPush1)) {
                    stack1.push(stackList.get(i));
                    operation.add("a");
//                    System.out.print("a ");
                    i++;
                } else if (stack2.size() == 0 || stack2.get(stack2.size() - 1) >= stackList.get(i)) {
                    stack2.push(stackList.get(i));
                    operation.add("c");
//                    System.out.print("c ");
                    i++;
                } else {
                    System.out.println("0");
                    return;
                }
            }
            if (i<size) {
                stack1.push(stackList.get(i++));
                operation.add("a");
                // System.out.print("a ");

            }
            // 如果找的到下一个数字则推出，如果找不到就结束继续
            while (!stack1.empty() || !stack2.empty()) {
                if (!stack1.empty()&&(result.size() == 0 || stack1.get(stack1.size() - 1) == result.get(result.size()-1)+1)) {
                    result.add(stack1.pop());
                    operation.add("b");
//                    System.out.print("b ");
//                System.out.print("b");
                } else if (!stack2.empty()&&stack2.get(stack2.size() - 1) == result.get(result.size()-1)+1) {
                    result.add(stack2.pop());
                    operation.add("d");
//                    System.out.print("d ");
//                System.out.print("d");
                } else{
                    break;
                }
            }
        }

        for (i = 0; i < operation.size(); i++) {
            if (i < operation.size() - 1) {
                System.out.print(operation.get(i) + " ");
            } else {
                System.out.print(operation.get(i));
            }
        }


    }
}