UVA12101 Prime Path

知识点：广搜，质数

比较简单的广搜，算是抽象的状态图的搜索，因为是最短次数，所以可以使用广搜

#include <bits/stdc++.h>

using namespace std;

const int N = 1e4 + 5;

int h[N], s, e;

void init() {
	for (int i = 2; i <= 9999; i++) {
		if (!h[i]) {
			for (int j = i + i; j <= 9999; j += i) {
				h[j] = 1;
			}
		}
	}
}

void bfs() {
	queue<int> q;
	q.push(s);
	int dist[N];
	memset(dist, -1, sizeof(dist));
	dist[s] = 0;
	while (!q.empty()) {
		int now = q.front(); q.pop();
		if (now == e) { cout << dist[e] << '\n'; return; }
		int x1 = 1, x2 = 10;
		for (int i = 0; i < 4; i++) {
			for (int j = 0; j < 10; j++) {
				int t = now / x2 * x2 + now % x1 + j * x1;
				if (t < 1000 || h[t] || dist[t] != -1) continue;
				q.push(t);
				dist[t] = dist[now] + 1;
			}
			x1 *= 10; x2 *= 10;
		}
	}
	cout << "Impossible\n";
}

int main() {
	init();
	int T;
	cin >> T;
	while (T--) {
		cin >> s >> e;
		bfs();
	}
	return 0;
}