Merge pull request #1812 from AlgorithmWithGod/JHLEE325

[20260122] BOJ / G3 / 소수 경로 / 이준희

Author: ShinHeeEul

JHLEE325/202601/22 BOJ G4 소수 경로.md

@@ -0,0 +1,80 @@
+```java
+import java.io.*;
+import java.util.*;
+
+public class Main {
+    static boolean[] isPrime = new boolean[10000];
+
+    public static void main(String[] args) throws IOException {
+        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+
+        Arrays.fill(isPrime, true);
+        isPrime[0] = isPrime[1] = false;
+        for (int i = 2; i * i < 10000; i++) {
+            if (isPrime[i]) {
+                for (int j = i * i; j < 10000; j += i) isPrime[j] = false;
+            }
+        }
+
+        int T = Integer.parseInt(br.readLine());
+        for (int t = 0; t < T; t++) {
+            StringTokenizer st = new StringTokenizer(br.readLine());
+            int start = Integer.parseInt(st.nextToken());
+            int end = Integer.parseInt(st.nextToken());
+
+            int result = bfs(start, end);
+            System.out.println(result == -1 ? "Impossible" : result);
+        }
+    }
+
+    static int bfs(int start, int end) {
+        Queue<Integer> q = new LinkedList<>();
+        int[] dist = new int[10000];
+        Arrays.fill(dist, -1);
+
+        q.add(start);
+        dist[start] = 0;
+
+        while (!q.isEmpty()) {
+            int curr = q.poll();
+            if (curr == end) return dist[curr];
+
+            for (int i = 0; i < 4; i++) {
+                int[] digits = getDigits(curr);
+
+                for (int d = 0; d <= 9; d++) {
+                    if (i == 3 && d == 0) continue;
+
+                    int next = changeDigit(digits, i, d);
+
+                    if (isPrime[next] && dist[next] == -1) {
+                        dist[next] = dist[curr] + 1;
+                        q.add(next);
+                    }
+                }
+            }
+        }
+        return -1;
+    }
+
+    static int[] getDigits(int num) {
+        int[] digits = new int[4];
+        for (int i = 0; i < 4; i++) {
+            digits[i] = num % 10;
+            num /= 10;
+        }
+        return digits;
+    }
+
+    static int changeDigit(int[] digits, int idx, int d) {
+        int res = 0;
+        int p = 1;
+        for (int i = 0; i < 4; i++) {
+            if (i == idx) res += d * p;
+            else res += digits[i] * p;
+            p *= 10;
+        }
+        return res;
+    }
+}
+```