feat: Add Dart Codes for section_iteration_and_recursion (#696)

codes/dart/chapter_computational_complexity/iteration.dart

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+/**
+ * File: iteration.dart
+ * Created Time: 2023-08-27
+ * Author: liuyuxin (gvenusleo@gmail.com)
+ */
+
+/* for 循环 */
+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 */
+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++; // 更新条件变量
+  }
+  return res;
+}
+
+/* while 循环（两次更新） */
+int whileLoopII(int n) {
+  int res = 0;
+  int i = 1; // 初始化条件变量
+  // 循环求和 1, 4, ...
+  while (i <= n) {
+    res += i;
+    // 更新条件变量
+    i++;
+    i *= 2;
+  }
+  return res;
+}
+
+/* 双层 for 循环 */
+String nestedForLoop(int n) {
+  String res = "";
+  // 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 += "($i, $j), ";
+    }
+  }
+  return res;
+}
+
+/* Driver Code */
+void main() {
+  int n = 5;
+  int res;
+
+  res = forLoop(n);
+  print("\nfor 循环的求和结果 res = $res");
+
+  res = whileLoop(n);
+  print("\nwhile 循环的求和结果 res = $res");
+
+  res = whileLoopII(n);
+  print("\nwhile 循环（两次更新）的求和结果 res = $res");
+
+  String resStr = nestedForLoop(n);
+  print("\n双层 for 循环的结果 $resStr");
+}

codes/dart/chapter_computational_complexity/recursion.dart

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+/**
+ * File: recursion.dart
+ * Created Time: 2023-08-27
+ * Author: liuyuxin (gvenusleo@gmail.com)
+ */
+
+/* 递归 */
+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 */
+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 */
+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 */
+void main() {
+  int n = 5;
+  int res;
+
+  res = recur(n);
+  print("\n递归函数的求和结果 res = $res");
+
+  res = tailRecur(n, 0);
+  print("\n尾递归函数的求和结果 res = $res");
+
+  res = fib(n);
+  print("\n斐波那契数列的第 $n 项为 $res");
+}