feat: iteration & recursion in Zig (#804)

* iteration & recursion in Zig

* missing part in time_complexity.md (zig)

* build.zig sync

* Update recursion.zig

* Update iteration.zig

---------

Co-authored-by: Yudong Jin <krahets@163.com>

codes/zig/chapter_computational_complexity/iteration.zig

@@ -0,0 +1,77 @@
+// File: iteration.zig
+// Created Time: 2023-09-27
+// Author: QiLOL (pikaqqpika@gmail.com)
+
+const std = @import("std");
+const Allocator = std.mem.Allocator;
+
+// for 循环
+fn forLoop(n: usize) i32 {
+    var res: i32 = 0;
+    // 循环求和 1, 2, ..., n-1, n
+    for (1..n+1) |i| {
+        res = res + @as(i32, @intCast(i));
+    }
+    return res;
+} 
+
+// while 循环
+fn whileLoop(n: i32) i32 {
+    var res: i32 = 0;
+    var i: i32 = 1; // 初始化条件变量
+    // 循环求和 1, 2, ..., n-1, n
+    while (i <= n) {
+        res += @intCast(i);
+        i += 1;
+    }
+    return res;
+}
+
+//  while 循环（两次更新）
+fn whileLoopII(n: i32) i32 {
+    var res: i32 = 0;
+    var i: i32 = 1; // 初始化条件变量
+    // 循环求和 1, 4, ...
+    while (i <= n) {
+        res += @intCast(i);
+        // 更新条件变量
+        i += 1;
+        i *= 2;
+    }
+    return res;
+}
+
+// 双层 for 循环
+fn nestedForLoop(allocator: Allocator, n: usize) ![]const u8 {
+    var res = std.ArrayList(u8).init(allocator);
+    defer res.deinit();
+    var buffer: [20]u8 = undefined;
+    // 循环 i = 1, 2, ..., n-1, n
+    for (1..n+1) |i| {
+        // 循环 j = 1, 2, ..., n-1, n
+        for (1..n+1) |j| {
+            var _str = try std.fmt.bufPrint(&buffer, "({d}, {d}), ", .{i, j});
+            try res.appendSlice(_str);
+        }
+    }
+    return res.toOwnedSlice();
+}
+
+// Driver Code
+pub fn main() !void {
+    const n: i32 = 5;
+    var res: i32 = 0;
+
+    res = forLoop(n);
+    std.debug.print("\nfor 循环的求和结果 res = {}\n", .{res});
+
+    res = whileLoop(n);
+    std.debug.print("\nwhile 循环的求和结果 res = {}\n", .{res});
+
+    res = whileLoopII(n);
+    std.debug.print("\nwhile 循环（两次更新）求和结果 res = {}\n", .{res});
+
+    const allocator = std.heap.page_allocator;
+    const resStr = try nestedForLoop(allocator, n);
+    std.debug.print("\n双层 for 循环的遍历结果 {s}\n", .{resStr});
+}