add zig codes for Section 'Space Complexity' and 'Space Time Tradeoff'

codes/zig/include/TreeNode.zig

@@ -0,0 +1,21 @@
+// File: TreeNode.zig
+// Created Time: 2023-01-07
+// Author: sjinzh (sjinzh@gmail.com)
+
+const std = @import("std");
+
+// Definition for a binary tree node
+pub fn TreeNode(comptime T: type) type {
+    return struct {
+        const Self = @This();
+
+        val: T = undefined,
+        left: ?*Self = null,
+        right: ?*Self = null,
+
+        // Initialize a tree node with specific value
+        pub fn init(self: *Self, x: i32) void {
+            self.val = x;
+        }
+    };   
+}