usaco1.52Prime Palindromes
dfs搜前5位数 根据前面的算后面的
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
1 /* 2 ID: your_id_here 3 PROG: pprime 4 LANG: C++ 5 */ 6 #include <iostream> 7 #include<cstdio> 8 #include<cstring> 9 #include<cmath> 10 #include<algorithm> 11 using namespace std; 12 int a,b,g; 13 int kk[10000],f[20][20],tt,ff[100000]; 14 char s[20]; 15 int prime(int x) 16 { 17 int i,j,k,flag = 1; 18 k = sqrt(x); 19 for(i = 2 ; i <= k ; i++) 20 { 21 if(x%i==0) 22 { 23 flag = 0; 24 break; 25 } 26 } 27 return flag; 28 } 29 int palind(int k,char *ss) 30 { 31 int i,flag = 1; 32 for(i = 1; i <= k ;i++) 33 if(ss[i]!=ss[k-i+1]) 34 { 35 flag = 0; 36 break; 37 } 38 return flag; 39 } 40 void dfs(int v) 41 { 42 int i,y,s1 =0,s2 =0,s3 = 0,j; 43 char str[20],str3[20]; 44 if(v>5) 45 return ; 46 for(i = 1; i <= v ; i++) 47 str[i] = s[i]; 48 for(i = 1; i <= v ; i++) 49 str3[i] = s[i]; 50 j = v; 51 for(i = v+1 ; i <= 2*v ; i++) 52 str[i] = s[j--]; 53 j = v-1; 54 for(i = v+1 ; i < 2*v ; i++) 55 str3[i] = s[j--]; 56 y = 1; 57 for(i = v; i >= 1 ; i--) 58 { 59 s1+=y*(s[i]-'0'); 60 y = y*10; 61 } 62 y = 1; 63 for(i = 2*v ; i >= 1 ; i--) 64 { 65 s2+=y*(str[i]-'0'); 66 y = y*10; 67 } 68 y = 1; 69 for(i = 2*v-1 ; i >= 1 ; i--) 70 { 71 s3+=y*(str3[i]-'0'); 72 y = y*10; 73 } 74 if(s1>=a&&s1<=b) 75 { 76 if(palind(v,s)&&prime(s1)) 77 { 78 if(s1<100000) 79 { 80 if(!ff[s1]) 81 { 82 g++; 83 kk[g] = s1; 84 ff[s1] = 1; 85 } 86 } 87 else 88 { 89 g++; 90 kk[g] = s1; 91 } 92 } 93 } 94 if(s2>=a&&s2<=b) 95 { 96 if(palind(2*v,str)&&prime(s2)) 97 { 98 if(s2<100000) 99 { 100 if(!ff[s2]) 101 { 102 g++; 103 kk[g] = s2; 104 ff[s2] = 1; 105 } 106 } 107 else 108 { 109 110 g++; 111 kk[g] = s2; 112 } 113 } 114 } 115 if(s3>=a&&s3<=b) 116 { 117 if(palind(2*v-1,str3)&&prime(s3)) 118 { 119 120 if(s3<100000) 121 { 122 if(!ff[s3]) 123 { 124 g++; 125 kk[g] = s3; 126 ff[s3] = 1; 127 } 128 } 129 else 130 { 131 g++; 132 kk[g] = s3; 133 } 134 } 135 } 136 if(s1>b) 137 return ; 138 for(i = 0 ; i <= 9 ; i++) 139 { 140 s[v+1] = i+'0'; 141 dfs(v+1); 142 } 143 } 144 int main() 145 { 146 freopen("pprime.in","r",stdin); 147 freopen("pprime.out","w",stdout); 148 int i; 149 char num[20]; 150 cin>>a>>b; 151 for(i = 1; i <= 9 ; i++) 152 { 153 s[1] = i+'0'; 154 dfs(1); 155 } 156 sort(kk+1,kk+g+1); 157 for(i = 1; i <= g ; i++) 158 cout<<kk[i]<<endl; 159 fclose(stdin); 160 fclose(stdout); 161 return 0; 162 }