雙向鏈表和普通鏈表的區(qū)別在于，在鏈表中，一個節(jié)點(diǎn)只有鏈向下一個節(jié)點(diǎn)的鏈接，而在雙向鏈表中，鏈接是雙向的：一個鏈向下一個元素，另一個鏈向前一個元素。

雙向鏈表提供了兩種迭代列表的方法：從頭到尾，或者反過來。我們也可以訪問一個特定節(jié)點(diǎn)的下一個或前一個元素。在單向鏈表中，如果迭代列表時錯過了要找的元素，就需要回到列表起點(diǎn)，重新開始迭代。這是雙向鏈表的一個優(yōu)點(diǎn)。

雙向鏈表：單向鏈表只能向著一個方向遍歷鏈表節(jié)點(diǎn)，而在節(jié)點(diǎn)指針域中增加了前向指針的雙向鏈表，則可以向著兩個方向遍歷節(jié)點(diǎn)。這使得雙向鏈表也可以在任何一個節(jié)點(diǎn)遍歷整個鏈表。

function DoublyLinkedList() {   var Node = function(element) {     this.element = element;     this.next = null;     this.prev = null;   };    var length = 0,     head = null,     tail = null;    this.append = function(element){     var node = Node(element),       current,       previous;          if(!head){       head = node;       tail = node;     }else{       current = head;       while(current){         previous = current;         current = current.next;       }        node.next = current;       current.prev = node;       previous.next = node;       node.prev = previous;     }      length++;     return true;   }    this.insert = function(position,element){     if(position > -1 && position < length){       var node = new Node(element),         current = head,         previous,         index = 0;        if(position === 0){          if(!head){           head = node;           tail = node;         }else{           node.next = current;           current.prev = node;           head = node;         }        }else if (position === length -1){         current = tail;         current.next = node;         node.prev = current;       }else {         while(index++ < position){           previous = current;           current = current.next;         }         node.next = current;         previous.next = node;         current.prev = node;         node.prev = previous;       }        length++;       return true;     }else{       return false;     }   };    this.removeAt = function(position){     if(position > -1 && position < length){       var current = head,         index = 0,         previous;        if (position === 0) {         head = current.next;          if(length === 1){           tail = null;         }else{           head.prev = null;         }       }else if(position === length - 1){         current = tail;         tail = current.prev;         tail.next = null;       } else{         while(index++ < position){           previous = current;           current = current.next;         }          previous.next = current.next;         current.next.prev = previous;       };       length-- ;        return current.element;     }else{       return false;     }   };    this.remove = function(element){     var current = head,       previous;      if(current.element === element){       head = current.next;     }     previous = current;     current = current.next;      while(current){       if (current.element = element) {         previous.next = current.next;         current.next.prev = previous;       }else{         previous = current;         current = current.next;       }     }     return false;   };    this.remove = function(){     if (length === 0) {       return false;     };      var current = head,       previous;      if(length === 1){       head = null;       tail = null;       length--;       return current.element;     }      while(current){       previous = current;       current = current.next;     }      previous.next = null;     length--;     return current.element;   };    this.indexOf = function(element){     var current = head,       index = 0;      while(current && index++ < length){       if (current.element === element) {         return index;       };       current = current.next;     }      return false;   };    this.isEmpty = function(){     return length === 0;   };    this.size = function(){     return length;   };    this.toString = function(){     var current = head,       string = '';      while(current){       string += current.element;       current = current.next;     }     return string;   };    this.getHead = function(){     return head;   };    this.getTail = function(){     return tail;   }; }