求区间回文素数，题意中文的不解释。
思路：
对于一个偶数位的回文数，必定是11的倍数，所以直接枚举奇数位，可以枚举 1位 3位 5位 7位 还有别忘记11这个数，最大的数是9989899所以不用9位，1 - 100000000里面一共有781个满足要求的数 <打表看出来的，嘿嘿>，这样打个素数表暴力就行了时间是578ms,如果不知道什么偶数位是11的倍数照样可以AC，而且感觉更省事，0ms,下面给出两种方法的代码。
非打表代码 578msAC
#include<stdio.h> #include<string.h> #include<stdlib.h> bool mark[9990000]; int ans[800]; void DB() { memset(mark ,0 ,sizeof(mark)); mark[1] = 1; for(int i = 2 ;i <= 9990000 ;i ++) { if(!mark[i]) for(int j = i * 2 ;j <= 9990000 ;j += i) mark[j] = 1; } int t = 0; for(int i = 1 ;i <= 11 ;i ++) { if(mark[i]) continue; int now[10] ,tt = 0 ,a = i; while(a) { now[++tt] = a % 10; a /= 10; } int mk = 0; for(int j = 1 ;j <= tt / 2 && !mk;j ++) if(now[j] != now[tt - j + 1])mk = 1; if(!mk) ans[++t] = i; } for(int i = 100 ;i <= 999 ;i ++) { if(mark[i]) continue; int now[10] ,tt = 0 ,a = i; while(a) { now[++tt] = a % 10; a /= 10; } int mk = 0; for(int j = 1 ;j <= tt / 2 && !mk;j ++) if(now[j] != now[tt - j + 1])mk = 1; if(!mk) ans[++t] = i; } for(int i = 10000 ;i <= 99999 ;i ++) { if(mark[i]) continue; int now[10] ,tt = 0 ,a = i; while(a) { now[++tt] = a % 10; a /= 10; } int mk = 0; for(int j = 1 ;j <= tt / 2 && !mk;j ++) if(now[j] != now[tt - j + 1])mk = 1; if(!mk) ans[++t] = i; } for(int i = 1000000 ;i <= 9990000 ;i ++) { if(mark[i]) continue; int now[10] ,tt = 0 ,a = i; while(a) { now[++tt] = a % 10; a /= 10; } int mk = 0; for(int j = 1 ;j <= tt / 2 && !mk;j ++) if(now[j] != now[tt - j + 1])mk = 1; if(!mk) ans[++t] = i; } } int main () { int a ,b; DB(); while(~scanf("%d %d" ,&a ,&b)) { for(int i = 1 ;i <= 781 ;i ++) if(ans[i] >= a && ans[i] <= b) printf("%d " ,ans[i]); printf(" "); } return 0; }
打表代码 0ms AC
#include<stdio.h> int mark[800] = { 2,3,5,7,11,101,131,151,181,191,313,353,373,383,727,757,787, 797,919,929,10301,10501,10601,11311,11411,12421,12721, 12821,13331,13831,13931,14341,14741,15451,15551,16061,16361, 16561,16661,17471,17971,18181,18481,19391,19891,19991,30103, 30203,30403,30703,30803,31013,31513,32323,32423,33533,34543, 34843,35053,35153,35353,35753,36263,36563,37273,37573,38083, 38183,38783,39293,70207,70507,70607,71317,71917,72227,72727, 73037,73237,73637,74047,74747,75557,76367,76667,77377,77477, 77977,78487,78787,78887,79397,79697,79997,90709,91019,93139, 93239,93739,94049,94349,94649,94849,94949,95959,96269,96469, 96769,97379,97579,97879,98389,98689,1003001,1008001,1022201, 1028201,1035301,1043401,1055501,1062601,1065601,1074701,1082801, 1085801,1092901,1093901,1114111,1117111,1120211,1123211,1126211, 1129211,1134311,1145411,1150511,1153511,1160611,1163611,1175711, 1177711,1178711,1180811,1183811,1186811,1190911,1193911,1196911, 1201021,1208021,1212121,1215121,1218121,1221221,1235321,1242421, 1243421,1245421,1250521,1253521,1257521,1262621,1268621,1273721, 1276721,1278721,1280821,1281821,1286821,1287821,1300031,1303031, 1311131,1317131,1327231,1328231,1333331,1335331,1338331,1343431, 1360631,1362631,1363631,1371731,1374731,1390931,1407041,1409041, 1411141,1412141,1422241,1437341,1444441,1447441,1452541,1456541, 1461641,1463641,1464641,1469641,1486841,1489841,1490941,1496941, 1508051,1513151,1520251,1532351,1535351,1542451,1548451,1550551, 1551551,1556551,1557551,1565651,1572751,1579751,1580851,1583851, 1589851,1594951,1597951,1598951,1600061,1609061,1611161,1616161, 1628261,1630361,1633361,1640461,1643461,1646461,1654561,1657561, 1658561,1660661,1670761,1684861,1685861,1688861,1695961,1703071, 1707071,1712171,1714171,1730371,1734371,1737371,1748471,1755571, 1761671,1764671,1777771,1793971,1802081,1805081,1820281,1823281, 1824281,1826281,1829281,1831381,1832381,1842481,1851581,1853581, 1856581,1865681,1876781,1878781,1879781,1880881,1881881,1883881, 1884881,1895981,1903091,1908091,1909091,1917191,1924291,1930391, 1936391,1941491,1951591,1952591,1957591,1958591,1963691,1968691, 1969691,1970791,1976791,1981891,1982891,1984891,1987891,1988891, 1993991,1995991,1998991,3001003,3002003,3007003,3016103,3026203, 3064603,3065603,3072703,3073703,3075703,3083803,3089803,3091903, 3095903,3103013,3106013,3127213,3135313,3140413,3155513,3158513, 3160613,3166613,3181813,3187813,3193913,3196913,3198913,3211123, 3212123,3218123,3222223,3223223,3228223,3233323,3236323,3241423, 3245423,3252523,3256523,3258523,3260623,3267623,3272723,3283823, 3285823,3286823,3288823,3291923,3293923,3304033,3305033,3307033, 3310133,3315133,3319133,3321233,3329233,3331333,3337333,3343433, 3353533,3362633,3364633,3365633,3368633,3380833,3391933,3392933, 3400043,3411143,3417143,3424243,3425243,3427243,3439343,3441443, 3443443,3444443,3447443,3449443,3452543,3460643,3466643,3470743, 3479743,3485843,3487843,3503053,3515153,3517153,3528253,3541453, 3553553,3558553,3563653,3569653,3586853,3589853,3590953,3591953, 3594953,3601063,3607063,3618163,3621263,3627263,3635363,3643463, 3646463,3670763,3673763,3680863,3689863,3698963,3708073,3709073, 3716173,3717173,3721273,3722273,3728273,3732373,3743473,3746473, 3762673,3763673,3765673,3768673,3769673,3773773,3774773,3781873, 3784873,3792973,3793973,3799973,3804083,3806083,3812183,3814183, 3826283,3829283,3836383,3842483,3853583,3858583,3863683,3864683, 3867683,3869683,3871783,3878783,3893983,3899983,3913193,3916193, 3918193,3924293,3927293,3931393,3938393,3942493,3946493,3948493, 3964693,3970793,3983893,3991993,3994993,3997993,3998993,7014107, 7035307,7036307,7041407,7046407,7057507,7065607,7069607,7073707, 7079707,7082807,7084807,7087807,7093907,7096907,7100017,7114117, 7115117,7118117,7129217,7134317,7136317,7141417,7145417,7155517, 7156517,7158517,7159517,7177717,7190917,7194917,7215127,7226227, 7246427,7249427,7250527,7256527,7257527,7261627,7267627,7276727, 7278727,7291927,7300037,7302037,7310137,7314137,7324237,7327237, 7347437,7352537,7354537,7362637,7365637,7381837,7388837,7392937, 7401047,7403047,7409047,7415147,7434347,7436347,7439347,7452547, 7461647,7466647,7472747,7475747,7485847,7486847,7489847,7493947, 7507057,7508057,7518157,7519157,7521257,7527257,7540457,7562657, 7564657,7576757,7586857,7592957,7594957,7600067,7611167,7619167, 7622267,7630367,7632367,7644467,7654567,7662667,7665667,7666667, 7668667,7669667,7674767,7681867,7690967,7693967,7696967,7715177, 7718177,7722277,7729277,7733377,7742477,7747477,7750577,7758577, 7764677,7772777,7774777,7778777,7782877,7783877,7791977,7794977, 7807087,7819187,7820287,7821287,7831387,7832387,7838387,7843487, 7850587,7856587,7865687,7867687,7868687,7873787,7884887,7891987, 7897987,7913197,7916197,7930397,7933397,7935397,7938397,7941497, 7943497,7949497,7957597,7958597,7960697,7977797,7984897,7985897, 7987897,7996997,9002009,9015109,9024209,9037309,9042409,9043409, 9045409,9046409,9049409,9067609,9073709,9076709,9078709,9091909, 9095909,9103019,9109019,9110119,9127219,9128219,9136319,9149419, 9169619,9173719,9174719,9179719,9185819,9196919,9199919,9200029, 9209029,9212129,9217129,9222229,9223229,9230329,9231329,9255529, 9269629,9271729,9277729,9280829,9286829,9289829,9318139,9320239, 9324239,9329239,9332339,9338339,9351539,9357539,9375739,9384839, 9397939,9400049,9414149,9419149,9433349,9439349,9440449,9446449, 9451549,9470749,9477749,9492949,9493949,9495949,9504059,9514159, 9526259,9529259,9547459,9556559,9558559,9561659,9577759,9583859, 9585859,9586859,9601069,9602069,9604069,9610169,9620269,9624269, 9626269,9632369,9634369,9645469,9650569,9657569,9670769,9686869, 9700079,9709079,9711179,9714179,9724279,9727279,9732379,9733379, 9743479,9749479,9752579,9754579,9758579,9762679,9770779,9776779, 9779779,9781879,9782879,9787879,9788879,9795979,9801089,9807089, 9809089,9817189,9818189,9820289,9822289,9836389,9837389,9845489, 9852589,9871789,9888889,9889889,9896989,9902099,9907099,9908099, 9916199,9918199,9919199,9921299,9923299,9926299,9927299,9931399, 9932399,9935399,9938399,9957599,9965699,9978799,9980899,9981899,9989899, }; int main () { int a ,b; while(~scanf("%d %d" ,&a ,&b)) { for(int i = 0 ;i <= 780 ;i ++) if(mark[i] >= a && mark[i] <= b) printf("%d " ,mark[i]); printf(" "); } return 0; }
在附上一个 打表用的代码
#include<stdio.h>
#include<string.h>
#include<stdlib.h>
bool mark[100000005];
void DB()
{
memset(mark ,0 ,sizeof(mark));
mark[1] = 1;
for(int i = 2 ;i <= 10000000 ;i ++)
{
for(int j = i * 2 ;j <= 10000000 ;j += i)
mark[j] = 1;
}
int sum = 0;
for(int i = 1 ;i <= 10000000 ;i ++)
{
if(mark[i]) continue;
int now[10];
int t = 0;
int a = i;
while(a)
{
now[++t] = a % 10;
a /= 10;
}
int mk = 0;
for(int j = 1 ;j <= t / 2 && !mk;j ++)
if(now[j] != now[t - j + 1])mk = 1;
if(!mk) sum ++ ,printf("%d," ,i);
}
//printf("%d
" ,sum);
}
int main ()
{
//freopen("1431.txt","r",stdin);
freopen("1413.txt","w",stdout);
DB();
getchar();
}