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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en/codes/java/chapter_computational_complexity/complexity_exercises.java

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+/**
+ * File: complexity_exercises.java
+ * Created Time: 2026-08-18
+ * Author: Hello Algo Team
+ */
+
+package chapter_computational_complexity;
+
+public class complexity_exercises {
+    /* Iterative summation */
+    static int sumIter(int n) {
+        int res = 0;
+        for (int i = 1; i <= n; i++) {
+            res += i;
+        }
+        return res;
+    }
+
+    /* Recursive summation */
+    static int sumRecur(int n) {
+        if (n == 1) {
+            return 1;
+        }
+        return n + sumRecur(n - 1);
+    }
+
+    /* Linear loop */
+    static int linearLoop(int n) {
+        int res = 0;
+        for (int i = 0; i < n; i++) {
+            res += i;
+        }
+        return res;
+    }
+
+    /* Quadratic loop */
+    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;
+    }
+
+    /* Logarithmic loop */
+    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;
+    }
+}

en/codes/java/chapter_divide_and_conquer/fast_power.java

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+/**
+ * File: fast_power.java
+ * Created Time: 2026-08-18
+ * Author: Hello Algo Team
+ */
+
+package chapter_divide_and_conquer;
+
+public class fast_power {
+    /* Exponentiation by squaring */
+    static int fastPow(int x, int n) {
+        if (n == 0) {
+            return 1;
+        }
+        int half = fastPow(x, n / 2);
+        if (n % 2 == 0) {
+            return half * half;
+        }
+        return half * half * x;
+    }
+
+    public static void main(String[] args) {
+        assert fastPow(7, 0) == 1;
+        assert fastPow(3, 5) == 243;
+        assert fastPow(2, 6) == 64;
+    }
+}