[20250815] BOJ / G5 / Fly me to the Alpha../ 이종환

Fly me to the Alpha Centauri

Author: 0224LJH

0224LJH/202508/14 BOJ Fly me to the Alpha Centauri.md

@@ -0,0 +1,70 @@
+```java
+import java.io.*;
+import java.util.*;
+
+public class Main {
+
+    static final double EPS = 1e-6;
+    static StringBuilder sb = new StringBuilder();
+
+    public static void main(String[] args) throws Exception {
+        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+        int size = Integer.parseInt(br.readLine());
+
+        for (int t = 0; t < size; t++) {
+            StringTokenizer st = new StringTokenizer(br.readLine());
+            double[] nums = new double[4];
+            for (int i = 0; i < 4; i++) nums[i] = Double.parseDouble(st.nextToken());
+
+            boolean ok = canMake24(nums);
+            sb.append(ok ? "YES" : "NO").append('\n');
+        }
+        System.out.print(sb.toString());
+    }
+
+    private static boolean canMake24(double[] arr) {
+        List<Double> list = new ArrayList<>(4);
+        for (double v : arr) list.add(v);
+        return dfs(list);
+    }
+
+    private static boolean dfs(List<Double> nums) {
+        int n = nums.size();
+        if (n == 1) {
+            return Math.abs(nums.get(0) - 24.0) < EPS;
+        }
+
+        // 두 수 선택해서 하나로 줄이기
+        for (int i = 0; i < n; i++) {
+            for (int j = i + 1; j < n; j++) {
+                double a = nums.get(i), b = nums.get(j);
+                List<Double> nextCandidates = new ArrayList<>(6);
+
+                // 덧셈/곱셈은 교환법칙이 있어 한 번만 추가
+                nextCandidates.add(a + b);
+                nextCandidates.add(a * b);
+
+                // 뺄셈/나눗셈은 순서가 중요하므로 양쪽 모두 시도
+                nextCandidates.add(a - b);
+                nextCandidates.add(b - a);
+                if (Math.abs(b) > EPS) nextCandidates.add(a / b);
+                if (Math.abs(a) > EPS) nextCandidates.add(b / a);
+
+                // 선택한 두 수를 제외한 나머지에 후보 결과를 붙여서 재귀
+                for (double cand : nextCandidates) {
+                    List<Double> next = new ArrayList<>(n - 1);
+                    for (int k = 0; k < n; k++) {
+                        if (k == i || k == j) continue;
+                        next.add(nums.get(k));
+                    }
+                    next.add(cand);
+
+                    if (dfs(next)) return true; // 해 찾으면 바로 종료
+                }
+            }
+        }
+        return false;
+    }
+}
+
+```