JavaScript双向链表和双向循环链表的实现

双向链表和普通链表的区别在于，在链表中，一个节点只有链向下一个节点的链接，而在双向链表中，链接是双向的：一个链向下一个元素，另一个链向前一个元素。 双向链表提供了两种迭代列表的方法：从头到尾，或者反过来。我们也可以访问一个特定节点的下一个或前一个元素。在单向链表中，如果迭代列表时错过了要找的元素，就需要回到列表起点，重新开始迭代。这是双向链表的一个优点。本文主要介绍JavaScript数据结构之双向链表和双向循环链表的实现。

双向链表：单向链表只能向着一个方向遍历链表节点，而在节点指针域中增加了前向指针的双向链表，则可以向着两个方向遍历节点。这使得双向链表也可以在任何一个节点遍历整个链表。

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, 
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    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; 
  }; 
}

双向循环链表：将双向链表的头尾指针相连，就构成了双向循环链表。这种链表从任意一个节点都可以同时向两个方向进行节点遍历，查询节点的速度也是最快的。

/*双向循环链表*/ 
function DoublyCircularLinkedList(){ 
  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 = new Node(element), 
      current, 
      previous; 
 
    if (!head) { 
      head = node; 
      tail = node; 
      head.prev = tail; 
      tail.next = head; 
    }else{ 
      current = head; 
 
      while(current.next !== head){ 
        previous = current; 
        current = current.next; 
      } 
 
      current.next = node; 
      node.next = head; 
      node.prev = current; 
    }; 
 
    length++; 
    return true; 
  }; 
 
  this.insert = function(position, element){ 
    if(position >= 0 && position <= length){ 
      var node = new Node(element), 
        index = 0, 
        current = head, 
          previous; 
 
      if(position === 0){ 
         
        if(!head){ 
 
          node.next = node; 
          node.tail = node; 
          head = node; 
          tail = node; 
 
        }else{ 
 
          current.prev = node; 
          node.next = current; 
          head = node; 
          node.prev = tail; 
 
        } 
         
      }else if(position === length){ 
        current = tail; 
 
        current.next = node; 
        node.prev = current; 
        tail = node; 
        node.next = head; 
      }else{ 
 
        while(index++ < position){ 
          previous = current; 
          current = current.next; 
        } 
 
        current.prev = node; 
        node.next = current; 
        previous.next = 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){ 
 
        current.next.previous = tail; 
        head = current.next; 
 
      }else if(position === length - 1){ 
 
        current = tail; 
 
        current.prev.next = head; 
        head.prev = current.prev; 
        tail = current.prev; 
      }else{ 
 
        while(index++ < position){ 
          previous = current; 
          current = current.next; 
        } 
 
        previous.next = current.next; 
        current.next.prev = previous; 
 
      } 
 
      length--; 
      return true; 
    }else{ 
      return false; 
    } 
  }; 
 
  this.remove = function(element){ 
    var current = head, 
      previous, 
      indexCheck = 0; 
 
    while(current && indexCheck < length){ 
      if(current.element === element){ 
        if(indexCheck === 0){ 
          current.next.prev = tail; 
          head = current.next; 
        }else{ 
          current.next.prev = previous; 
          previous.next = current.next; 
        } 
        length--; 
        return true; 
      } 
 
      previous = current; 
      current = current.next; 
      indexCheck++; 
    } 
 
    return false; 
  }; 
 
  this.remove = function(){ 
    if(length === 0){ 
      return false; 
    } 
 
    var current = head, 
      previous, 
      indexCheck = 0; 
 
    if(length === 1){ 
      head = null; 
      tail = null; 
      length--; 
      return current.element; 
    } 
 
    while(indexCheck++ < length){ 
      previous = current; 
      current = current.next; 
    } 
 
    previous.next = head; 
    tail = previous.next; 
    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.toString = function(){ 
    var current = head, 
      indexCheck = 0, 
      string = ''; 
 
    while(current && indexCheck < length){ 
      string += current.element; 
      indexCheck++; 
      current = current.next; 
    }   
 
    return string; 
  }; 
 
  this.isEmpty = function(){ 
    return length === 0; 
  }; 
 
  this.getHead = function(){ 
    return head; 
  }; 
 
  this.getTail = function(){ 
    return tail; 
  }; 
 
  this.size = function(){ 
    return length; 
  }; 
}

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