题目传送门
1 /*
2 题意：从一个数到另外一个数，每次改变一个数字，且每次是素数
3 BFS：先预处理1000到9999的素数，简单BFS一下。我没输出Impossible都AC，数据有点弱
4 */
5 /************************************************
6 Author :Running_Time
7 Created Time :2015-8-2 15:46:57
8 File Name :POJ_3126.cpp
9 *************************************************/
10
11 #include <cstdio>
12 #include <algorithm>
13 #include <iostream>
14 #include <sstream>
15 #include <cstring>
16 #include <cmath>
17 #include <string>
18 #include <vector>
19 #include <queue>
20 #include <deque>
21 #include <stack>
22 #include <list>
23 #include <map>
24 #include <set>
25 #include <bitset>
26 #include <cstdlib>
27 #include <ctime>
28 using namespace std;
29
30 #define lson l, mid, rt << 1
31 #define rson mid + 1, r, rt << 1 | 1
32 typedef long long ll;
33 const int MAXN = 1e4 + 10;
34 const int INF = 0x3f3f3f3f;
35 const int MOD = 1e9 + 7;
36 bool is_prime[MAXN];
37 bool vis[MAXN];
38 int x, y, res;
39 struct Digit {
40 int d[5];
41 int step;
42 };
43
44 void solve(void) {
45 memset (is_prime, true, sizeof (is_prime));
46 for (int i=1000; i<=9999; ++i) {
47 for (int j=2; j<i; ++j) {
48 if (i % j == 0) {
49 is_prime[i] = false; break;
50 }
51 }
52 }
53 }
54
55 int get_num(int *a) {
56 int ret = 0;
57 for (int i=1; i<=4; ++i) {
58 ret = ret * 10 + a[i];
59 }
60 return ret;
61 }
62
63 void BFS(int u, int v) {
64 Digit tmp;
65 for (int i=4; i>=1; --i) {
66 tmp.d[i] = u % 10; u /= 10;
67 }
68 tmp.step = 0;
69 queue<Digit> Q; Q.push (tmp); vis[u] = true;
70 int ans = -1;
71 while (!Q.empty ()) {
72 Digit x = Q.front (); Q.pop ();
73 int m = get_num (x.d);
74 if (m == v) {
75 ans = x.step; break;
76 }
77 for (int i=1; i<=4; ++i) {
78 for (int j=0; j<=9; ++j) {
79 if (i == 1 && j == 0) continue;
80 if (x.d[i] != j) {
81 Digit y = x;
82 y.d[i] = j; m = get_num (y.d);
83 if (is_prime[m] && !vis[m]) {
84 vis[m] = true; y.step++; Q.push (y);
85 }
86 }
87 }
88 }
89 }
90 if (ans == -1) puts ("Impossible");
91 else printf ("%d
", ans);
92 }
93
94 int main(void) { //POJ 3126 Prime Path
95 solve ();
96 int T; scanf ("%d", &T);
97 while (T--) {
98 scanf ("%d%d", &x, &y);
99 memset (vis, false, sizeof (vis));
100 BFS (x, y);
101 }
102
103 return 0;
104 }