add zig codes for Section 'Hash Map' and 'Linear Search'

codes/zig/chapter_searching/linear_search.zig

@@ -47,7 +47,8 @@ pub fn main() !void {
     const mem_allocator = mem_arena.allocator();
     var head = try inc.ListUtil.listToLinkedList(i32, mem_allocator, nums);
     var node = linearSearchLinkedList(i32, head, target);
-    std.debug.print("目标结点值 3 的对应结点对象为 {any}", .{node});
+    std.debug.print("目标结点值 3 的对应结点对象为 ", .{});
+    try inc.PrintUtil.printLinkedList(i32, node);
 
     const getchar = try std.io.getStdIn().reader().readByte();
     _ = getchar;

codes/zig/include/ListNode.zig

@@ -16,6 +16,7 @@ pub fn ListNode(comptime T: type) type {
         // Initialize a list node with specific value
         pub fn init(self: *Self, x: i32) void {
             self.val = x;
+            self.next = null;
         }
     };
 }

codes/zig/include/TreeNode.zig

@@ -17,6 +17,8 @@ pub fn TreeNode(comptime T: type) type {
         // Initialize a tree node with specific value
         pub fn init(self: *Self, x: i32) void {
             self.val = x;
+            self.left = null;
+            self.right = null;
         }
     };   
 }