function Tree() {

     this.root = null;

   }

   Tree.prototype = {

     constructor: Tree,

     addItem: function(value) {

       var Node = {

         data: value,

         left: null,

         right: null

       };

       if (this.root == null) {

         this.root = Node;

       } else {

         var current = this.root;

         var parent = current;

         while (current !== null) {

           parent = current;

           if (value < current.data) {

             current = current.left;

             continue; //此处easy忽略，缺少下一句if推断current.data会报错

           }

           if (value === current.data) {

             return false;

           }

           if (value > current.data) {

             current = current.right;

             continue;

           }

         }

         if (value < parent.data) {

           parent.left = Node;

         }

         if (value > parent.data) {

           parent.right = Node;

         }

       }

     },

     /*先序遍历*/

     firstlist: function(root) {

       if (root !== null) {

         console.log(root.data);

         this.firstlist(root.left);

         this.firstlist(root.right);

       }

     },

     /*后序遍历*/

     lastlist: function(root) {

       if (root !== null) {

         this.lastlist(root.left);

         this.lastlist(root.right);

         console.log(root.data);

       }

     },

     /*中序遍历*/

     inlist: function(root) {

       if (root !== null) {

         this.inlist(root.left);

         console.log(root.data);

         this.inlist(root.right);

       }

     }

   };

   var Tree = new Tree();

   Tree.addItem(5);

   Tree.addItem(1);

   Tree.addItem(6);

   Tree.addItem(8);

   Tree.addItem(7);