hdu1016枚举递归-经典DFS-简单回溯入门

1）注意格式要求(@1234)；先顺时针输出再逆时针输出并不用存储所有可能再倒着输出一遍，因为逆时针的输出被包含在前者之中，这里我应该是没读懂题意；之前在想怎么运行完一次之后或者找不到之后如何退回到上一步，是不是要return什么啊之类的，其实执行完递归就自动退回了，如果符合边界条件就输出，不符合就继续执行，然后自动退回到上一层，在上一层这个递归语句之后只是需要将相应的“标记数组”之类的，或者cur当前所指，减一即可，回溯的真谛、dfs的真谛，还是理解不透彻；另外需要递归时，确定递归函数参数，然后在递归函数里写两个分支，即if{};（//边界条件）else{};即可。

#include <iostream>
#include <math.h>
#include <string.h>

using namespace std;
int n;int g;
int kase;
int bj[23]={0};
int jg[23];
//int jilu[23][23];
int pri[41];//={1},youxiao ma
void dfs(int cur)
{
if(cur==n&&pri[jg[n-1]+jg[0]]){
for(int i=0;i<n-1;i++){
cout<<jg[i]<<" ";//@2
}
cout<<jg[n-1]<<endl;//@3
//memcpy(&jg,&jilu[g++],sizeof(jg));
}
else{
for(int i=2;i<=n;i++){
if(!bj[i]&&pri[jg[cur-1]+i]){
bj[i]=1;
jg[cur]=i;
dfs(cur+1);//不用cur++，则从这递归完成出来以后也不用cur--；
bj[i]=0;
}
}
}
}
bool is_pri(int k){
for(int i=2;i<=sqrt(k);i++){
if(k%i==0){
return 0;
}
}
return 1;
}
int main()
{
//素数提前打表，提高速度
for(int i=2;i<=40;i++){
if(is_pri(i))
pri[i]=1;
}
kase=0;
while(cin>>n){
printf("Case %d:\n",++kase);//@1
jg[0]=1;
dfs(1);
cout<<endl;//@4
//prin();
}
}

2）

Prime Ring Problem

Time Limit: 4000/2000 MS (Java/Others) Memory Limit: 65536/32768 K (Java/Others)

Total Submission(s): 38645 Accepted Submission(s): 17074



Problem Description

A ring is compose of n circles as shown in diagram. Put natural number 1, 2, ..., n into each circle separately, and the sum of numbers in two adjacent circles should be a prime.

Note: the number of first circle should always be 1.



Input

n (0 < n < 20).

Output

The output format is shown as sample below. Each row represents a series of circle numbers in the ring beginning from 1 clockwisely and anticlockwisely. The order of numbers must satisfy the above requirements. Print solutions in lexicographical order.

You are to write a program that completes above process.

Print a blank line after each case.

Sample Input

6
8

Sample Output

Case 1:
1 4 3 2 5 6
1 6 5 2 3 4

Case 2:
1 2 3 8 5 6 7 4
1 2 5 8 3 4 7 6
1 4 7 6 5 8 3 2
1 6 7 4 3 8 5 2

Source

Asia 1996, Shanghai (Mainland China)