用树形DP做了一下，感觉要比DFS凄惨很多，附2000以内的素数表，共303个：
const
pl:array[1..303]of longint=(
2,3,5,7,11,13,17,19,23,29,
31,37,41,43,47,53,59,61,67,71,
73,79,83,89,97,101,103,107,109,113,
127,131,137,139,149,151,157,163,167,173,				179,181,191,193,197,199,211,223,227,229,				233,239,241,251,257,263,269,271,277,281,				283,293,307,311,313,317,331,337,347,349,				353,359,367,373,379,383,389,397,401,409,				419,421,431,433,439,443,449,457,461,463,				467,479,487,491,499,503,509,521,523,541,				547,557,563,569,571,577,587,593,599,601,				607,613,617,619,631,641,643,647,653,659,				661,673,677,683,691,701,709,719,727,733,				739,743,751,757,761,769,773,787,797,809,				811,821,823,827,829,839,853,857,859,863,				877,881,883,887,907,911,919,929,937,941,				947,953,967,971,977,983,991,997,1009,1013,				1019,1021,1031,1033,1039,1049,1051,1061,1063,1069,			1087,1091,1093,1097,1103,1109,1117,1123,1129,1151,			1153,1163,1171,1181,1187,1193,1201,1213,1217,1223,			1229,1231,1237,1249,1259,1277,1279,1283,1289,1291,			1297,1301,1303,1307,1319,1321,1327,1361,1367,1373,			1381,1399,1409,1423,1427,1429,1433,1439,1447,1451,			1453,1459,1471,1481,1483,1487,1489,1493,1499,1511,			1523,1531,1543,1549,1553,1559,1567,1571,1579,1583,			1597,1601,1607,1609,1613,1619,1621,1627,1637,1657,			1663,1667,1669,1693,1697,1699,1709,1721,1723,1733,			1741,1747,1753,1759,1777,1783,1787,1789,1801,1811,			1823,1831,1847,1861,1867,1871,1873,1877,1879,1889,		1901,1907,1913,1931,1933,1949,1951,1973,1979,1987，			1993,1997,1999);
maxp=303;