HDU 1627 Krypton Factor

回溯法：避免无用判断，强化回溯代码的实现过程

题目的大意就是以字典序为准，排列字符串，但要保证一个字符串中不包含相邻的重复子串。

Problem Description

For example, the sequence ABACBCBAD is easy, since it contains an adjoining repetition of the subsequence CB. Other examples of easy sequences are:

BB
ABCDACABCAB
ABCDABCD

Some examples of hard sequences are:

D
DC
ABDAB
CBABCBA

Input

In order to provide the Quiz
Master with a potentially unlimited source of questions you are asked to write
a program that will read input lines that contain integers n and L (in that
order), where n > 0 and L is in the range , and for each input line prints
out the nth hard sequence (composed of letters drawn from the first L letters
in the alphabet), in increasing alphabetical order (alphabetical ordering here
corresponds to the normal ordering encountered in a dictionary), followed (on
the next line) by the length of that sequence. The first sequence in this
ordering is A. You may assume that for given n and L there do exist at least n
hard sequences.

For example, with L = 3, the first 7 hard sequences are:

A
AB
ABA
ABAC
ABACA
ABACAB
ABACABA
As each sequence is potentially very long, split it into groups of four (4)
characters separated by a space. If there are more than 16 such groups, please
start a new line for the 17th group.

Therefore, if the integers 7 and 3 appear on an input line, the output lines
produced should be

ABAC ABA
7
Input is terminated by a line containing two zeroes. Your program may assume a
maximum sequence length of 80.

Sample Input

30 3

0 0

Sample Output

ABAC ABCA CBAB CABA CABC ACBA
CABA

28

在判断当前字符串是否已经存在连续的重复子串，例如判断ABACABA是否包含连续重复子串，并不需要说检查该字符串所有长度为偶数的子串，仅需要做的是判断当前串的后缀。

另外对回溯法来说，一旦本次所要赋值的字符不满足，并且在循环了所有可以在这一次赋值的字符后，仍未满足，就要回溯到上一层，这种情况首先要保证成功次数不要增加（可以将次数设置为全局变量，当然进行完这一组数据后，要记得重新赋值，避免后面的测试数据出现问题，通常全局变量都要在完成一组数据测试后，重新赋初始值）

代码如下

#include<iostream>
#include<cstdio>
#include<cstring>
#define MAXN 200
using namespace std;
char alp[MAXN]="AABCDEFGHIJKLMNOPQRSTUVWXYZ";//首先打表方便后面的操作
char ch[MAXN]="#";//处理一个小的细节，当第一个字符时，其实不需要判断前面的字符，为了统一操作，在ch[0]处赋一个与第一个字符一定不同的量
int c=;
bool tag=false;//全局变量，完成字符串的标志
void dfs(int n,int t,int cur)
{
    int i,j,k=,m,s;
    bool flag=true;
    if(n==c)//以次数作为完成的标志
    {
        while(k<=cur-)
        {
            if(k==)
                cout<<endl;
            if(k%==&&k!=&&k!=cur-)//处理格式要求
                cout<<ch[k]<<' ';
            else
                cout<<ch[k];
            k++;
        }
        cout<<endl;
        cout<<cur-<<endl;
        tag=true;//以标示符作为字符串完成的标志
    }
    else
    {
        for(i=;i<=t;i++)//循环此次可以赋值的字符
        {
            flag=true;
            ch[cur]=alp[i];
            for(j=cur/;j<=cur-;j++)//字符串判重
            {
                s=;
                m=cur-j-;
                for(k=;k<=m;k++)
                {
                    if(ch[cur-k]==ch[j-k])
                        s++;
                }
                if(s==m+)
                {
                    flag=false;
                    break;
                }
            }
            if(flag==false)
                continue;
            if(!tag)
            {
                c++;
                dfs(n,t,cur+);//满足，则字符的长度加一，这也满足字典序的要求，若回溯到这里，那么即回头继续循环可以满足的字符，相当于说字符长度加一个不满足，那么前一个字符往后改，就如同从ABC->ABD(原本要走ABCD的)
            }
            if(tag)
            {
              //  cout<<"return ";
                return ;//若已完成则一步步回头
            }
        }
    }
}
int main()
{
    int n,l;
    while(cin>>n>>l)
    {
        if(n==&&l==)
            break;
        else
            dfs(n,l,);
        tag=false;//全局变量重新赋初始值
        c=;// 全局变量重新赋初始值
    }
    return ;
}