(拓扑排序11.3.1)POJ 1270 Following Orders(拓扑排序: 求变量串在约束组的作用下产生的有序集)

package com.njupt.acm;

import java.util.Scanner;

public class POJ_1270 {

	public static int pre[];//入度序列
	public static boolean has[];//标志序列
	public static int N;//节点数
	public static String var;//变量串
	public static String v;//约束组串
	
	public static void dfs(int dep,String res){//从长度为dep-1 的子序列res出发计算拓扑序列
		if(dep == N + 1){
			System.out.println(res);
			return ;
		}
		
		for(int i = 'a' ; i <= 'z' ; ++i){
			if(has[i] && pre[i] == 0){
				has[i] = false;
				for(int k = 0 ; k < v.length() ; k += 4){//删除约束组中所有入度为0的点的出边
					if(v.charAt(k) == i){
						--pre[v.charAt(k+2)];
					}
				}
				
				dfs(dep+1,res+((char)i));
				
				for(int k = 0 ; k < v.length() ; k+=4){
					if(v.charAt(k) == i){
						++pre[v.charAt(k+2)];
					}
				}
				has[i] = true;
			}
		}
	}
	
	public static void main(String[] args) {
		Scanner scanner = new Scanner(System.in);
		while(scanner.hasNextLine()){
			pre = new int[1 << 8];
			has = new boolean[1 << 8];
			
			var = scanner.nextLine();
			v = scanner.nextLine();
			N = var.length()/2 + 1;//计算总结点数
			
			for(int i = 0 ; i < var.length() ; i += 2){
				has[var.charAt(i)] = true;
			}
			
			for(int i = 0 ; i < v.length() ; i += 4){//在每对约束组x y中,x的出度为1,y的入度为1
				++pre[v.charAt(i+2)];
			}
			
			dfs(1,"");
			System.out.println();
		}
	}
}