add zig codes for Section Binary Tree, Binary Search Tree and AVL Tree (#293)

* add zig codes for Section 'Binary Tree'

* add zig codes for Section 'Binary Tree'

* add zig codes for Section 'Binary Tree'

* add zig codes for Section 'Binary Tree'

* add zig codes for Section 'Binary Tree' and 'Binary Search Tree'

* update zig codes for Section 'Binary Tree' and 'Binary Search Tree'

* update zig codes for Section 'Binary Tree', 'Binary Search Tree' and 'AVL Tree'

codes/zig/chapter_tree/binary_tree_dfs.zig

@@ -0,0 +1,70 @@
+// File: binary_tree_dfs.zig
+// Created Time: 2023-01-15
+// Author: sjinzh (sjinzh@gmail.com)
+
+const std = @import("std");
+const inc = @import("include");
+
+var list = std.ArrayList(i32).init(std.heap.page_allocator);
+
+// 前序遍历
+fn preOrder(comptime T: type, root: ?*inc.TreeNode(T)) !void {
+    if (root == null) return;
+    // 访问优先级：根结点 -> 左子树 -> 右子树
+    try list.append(root.?.val);
+    try preOrder(T, root.?.left);
+    try preOrder(T, root.?.right);
+}
+
+// 中序遍历
+fn inOrder(comptime T: type, root: ?*inc.TreeNode(T)) !void {
+    if (root == null) return;
+    // 访问优先级：左子树 -> 根结点 -> 右子树
+    try inOrder(T, root.?.left);
+    try list.append(root.?.val);
+    try inOrder(T, root.?.right);
+}
+
+// 后序遍历
+fn postOrder(comptime T: type, root: ?*inc.TreeNode(T)) !void {
+    if (root == null) return;
+    // 访问优先级：左子树 -> 右子树 -> 根结点
+    try postOrder(T, root.?.left);
+    try postOrder(T, root.?.right);
+    try list.append(root.?.val);
+}
+
+// Driver Code
+pub fn main() !void {
+    // 初始化内存分配器
+    var mem_arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
+    defer mem_arena.deinit();
+    const mem_allocator = mem_arena.allocator();
+
+    // 初始化二叉树
+    // 这里借助了一个从数组直接生成二叉树的函数
+    var nums = [_]i32{1, 2, 3, 4, 5, 6, 7};
+    var root = try inc.TreeUtil.arrToTree(i32, mem_allocator, &nums);
+    std.debug.print("初始化二叉树\n", .{});
+    try inc.PrintUtil.printTree(root, null, false);
+
+    // 前序遍历
+    list.clearRetainingCapacity();
+    try preOrder(i32, root);
+    std.debug.print("\n前序遍历的结点打印序列 = ", .{});
+    inc.PrintUtil.printList(i32, list);
+
+    // 中序遍历
+    list.clearRetainingCapacity();
+    try inOrder(i32, root);
+    std.debug.print("\n中序遍历的结点打印序列 = ", .{});
+    inc.PrintUtil.printList(i32, list);
+
+    // 后序遍历
+    list.clearRetainingCapacity();
+    try postOrder(i32, root);
+    std.debug.print("\n后续遍历的结点打印序列 = ", .{});
+    inc.PrintUtil.printList(i32, list);
+
+    _ = try std.io.getStdIn().reader().readByte();
+}