Add multilingual exercise code (#1959)

Replace the exercise pages' Python-only snippets with source-backed implementations for the 13 visible programming languages, and localize reader-facing comments by site language. Zig remains out of scope.

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ja/codes/java/chapter_computational_complexity/complexity_exercises.java

@@ -0,0 +1,62 @@
+/**
+ * File: complexity_exercises.java
+ * Created Time: 2026-08-18
+ * Author: Hello Algo Team
+ */
+
+package chapter_computational_complexity;
+
+public class complexity_exercises {
+    /* 反復による総和 */
+    static int sumIter(int n) {
+        int res = 0;
+        for (int i = 1; i <= n; i++) {
+            res += i;
+        }
+        return res;
+    }
+
+    /* 再帰による総和 */
+    static int sumRecur(int n) {
+        if (n == 1) {
+            return 1;
+        }
+        return n + sumRecur(n - 1);
+    }
+
+    /* 線形時間のループ */
+    static int linearLoop(int n) {
+        int res = 0;
+        for (int i = 0; i < n; i++) {
+            res += i;
+        }
+        return res;
+    }
+
+    /* 二次時間のループ */
+    static int quadraticLoop(int n) {
+        int res = 0;
+        for (int i = 0; i < n; i++) {
+            for (int j = i; j < n; j++) {
+                res += j;
+            }
+        }
+        return res;
+    }
+
+    /* 対数時間のループ */
+    static int logarithmicLoop(int n) {
+        while (n > 1) {
+            n /= 2;
+        }
+        return n;
+    }
+
+    public static void main(String[] args) {
+        assert sumIter(1) == 1 && sumRecur(1) == 1;
+        assert sumIter(4) == 10 && sumRecur(4) == 10;
+        assert linearLoop(4) == 6;
+        assert quadraticLoop(4) == 20;
+        assert logarithmicLoop(4) == 1 && logarithmicLoop(5) == 1;
+    }
+}