function RTImage(root,path,list) {
  this.root = document.getElementById(root);
  this.path = path;
  this.list = list;
  this.index = 0;
}
RTImage.prototype.prev = function() {
  this.index = (this.index+this.list.length-1)%this.list.length;
  this.root.src = this.path+this.list[this.index];
}
RTImage.prototype.next = function() {
  this.index = (this.index+1)%this.list.length;
  this.root.src = this.path+this.list[this.index];
}