js无限极分类

<script>      var arr = [        {id:1,city:"水果",parent_id:0},        {id:2,city:"蔬菜",parent_id:0},        {id:3,city:"苹果",parent_id:1},        {id:4,city:"香菜",parent_id:2},      ];      var treeArr = [];      function tree( arr , pid , level){        for (var i = 0; i < arr.length; i++) {          if( arr[i].parent_id == pid ){              arr[i].level = level;              treeArr.push(arr[i]);              tree( arr,arr[i].id,level+1 );          }        }      }      tree( arr , 0 , 0);    for (var i = 0; i < treeArr.length; i++) {      var tmpStr = "";      for (var j = 0; j < treeArr[i].level; j++) {          tmpStr += "--";      }      document.write( tmpStr  + treeArr[i].city + "<br/>");    }  </script>