Add the section of iteration and recursion. (#693)

codes/java/chapter_computational_complexity/iteration.java

@@ -0,0 +1,76 @@
+/**
+ * File: iteration.java
+ * Created Time: 2023-08-24
+ * Author: Krahets (krahets@163.com)
+ */
+
+package chapter_computational_complexity;
+
+public class iteration {
+    /* for 循环 */
+    public static int forLoop(int n) {
+        int res = 0;
+        // 循环求和 1, 2, ..., n-1, n
+        for (int i = 1; i <= n; i++) {
+            res += i;
+        }
+        return res;
+    }
+
+    /* while 循环 */
+    public static int whileLoop(int n) {
+        int res = 0;
+        int i = 1; // 初始化条件变量
+        // 循环求和 1, 2, ..., n-1, n
+        while (i <= n) {
+            res += i;
+            i += 1; // 更新条件变量
+        }
+        return res;
+    }
+
+    /* while 循环（两次更新） */
+    public static int whileLoopII(int n) {
+        int res = 0;
+        int i = 1; // 初始化条件变量
+        // 循环求和 1, 2, 4, 5...
+        while (i <= n) {
+            res += i;
+            i += 1; // 更新条件变量
+            res += i;
+            i *= 2; // 更新条件变量
+        }
+        return res;
+    }
+
+    /* 双层 for 循环 */
+    public static String nestedForLoop(int n) {
+        StringBuilder res = new StringBuilder();
+        // 循环 i = 1, 2, ..., n-1, n
+        for (int i = 1; i <= n; i++) {
+            // 循环 j = 1, 2, ..., n-1, n
+            for (int j = 1; j <= n; j++) {
+                res.append("(" + i + ", " + j + "), ");
+            }
+        }
+        return res.toString();
+    }
+
+    /* Driver Code */
+    public static void main(String[] args) {
+        int n = 5;
+        int res;
+
+        res = forLoop(n);
+        System.out.println("\nfor 循环的求和结果 res = " + res);
+
+        res = whileLoop(n);
+        System.out.println("\nwhile 循环的求和结果 res = " + res);
+
+        res = whileLoopII(n);
+        System.out.println("\nwhile 循环（两次更新）求和结果 res = " + res);
+
+        String resStr = nestedForLoop(n);
+        System.out.println("\n双层 for 循环的遍历结果 " + resStr);
+    }
+}

codes/java/chapter_computational_complexity/recursion.java

@@ -0,0 +1,55 @@
+/**
+ * File: recursion.java
+ * Created Time: 2023-08-24
+ * Author: Krahets (krahets@163.com)
+ */
+
+package chapter_computational_complexity;
+
+public class recursion {
+    /* 递归 */
+    public static int recur(int n) {
+        // 终止条件
+        if (n == 1)
+            return 1;
+        // 递：递归调用
+        int res = recur(n - 1);
+        // 归：返回结果
+        return n + res;
+    }
+
+    /* 尾递归 */
+    public static int tailRecur(int n, int res) {
+        // 终止条件
+        if (n == 0)
+            return res;
+        // 尾递归调用
+        return tailRecur(n - 1, res + n);
+    }
+
+    /* 斐波那契数列：递归 */
+    public static int fib(int n) {
+        // 终止条件 f(1) = 0, f(2) = 1
+        if (n == 1 || n == 2)
+            return n - 1;
+        // 递归调用 f(n) = f(n-1) + f(n-2)
+        int res = fib(n - 1) + fib(n - 2);
+        // 返回结果 f(n)
+        return res;
+    }
+
+    /* Driver Code */
+    public static void main(String[] args) {
+        int n = 5;
+        int res;
+
+        res = recur(n);
+        System.out.println("\n递归函数的求和结果 res = " + res);
+
+        res = tailRecur(n, 0);
+        System.out.println("\n尾递归函数的求和结果 res = " + res);
+
+        res = fib(n);
+        System.out.println("\n斐波那契数列的第 " + n + " 项为 " + res);
+    }
+}