算法|203. 移除链表元素 206.反转链表 707. 设计链表

链表

关于链表这块还是不太熟悉， 形似数组，却和数组不一样

203. 移除链表元素

while一直循环当前， 循环模块改变cur到下一个

/**
 * @param {ListNode} head
 * @param {number} val
 * @return {ListNode}
 */
var removeElements = function (head, val) {
  const ret = new ListNode(0, head);
  let cur = ret;
  while (cur.next) {
    if (cur.next.val === val) {
      cur.next = cur.next.next;
      // 继续下一个循环
      continue;
    }
    cur = cur.next;
  }
  return ret.next;
};
console.log(removeElements([7, 7, 7, 7], 7));

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206.反转链表

这个还算好理解，建几个临时变量， 改变pre cur next的值

/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var reverseList = function (head) {
  let temp = null;
  let pre = null;
  let cur = head;
  while (cur) {
    // 下一个存储起来
    temp = cur.next;
    // 当前值的next指向 pre
    cur.next = pre;
    // 将当前值给pre
    pre = cur;
    // 改变当前值为下一个,继续循环
    cur = temp;
  }
  return pre;
};

console.log(reverseList([1, 2, 3, 4, 5]));
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707. 设计链表

内容较多， 后续补上

class LinkNode {
    constructor(val, next) {
        this.val = val;
        this.next = next;
    }
}

/**
 * Initialize your data structure here.
 * 单链表 储存头尾节点 和 节点数量
 */
var MyLinkedList = function() {
    this._size = 0;
    this._tail = null;
    this._head = null;
};

/**
 * Get the value of the index-th node in the linked list. If the index is invalid, return -1. 
 * @param {number} index
 * @return {number}
 */
MyLinkedList.prototype.getNode = function(index) {
    if(index < 0 || index >= this._size) return null;
    // 创建虚拟头节点
    let cur = new LinkNode(0, this._head);
    // 0 -> head
    while(index-- >= 0) {
        cur = cur.next;
    }
    return cur;
};
MyLinkedList.prototype.get = function(index) {
    if(index < 0 || index >= this._size) return -1;
    // 获取当前节点
    return this.getNode(index).val;
};

/**
 * Add a node of value val before the first element of the linked list. After the insertion, the new node will be the first node of the linked list. 
 * @param {number} val
 * @return {void}
 */
MyLinkedList.prototype.addAtHead = function(val) {
    const node = new LinkNode(val, this._head);
    this._head = node;
    this._size++;
    if(!this._tail) {
        this._tail = node;
    }
};

/**
 * Append a node of value val to the last element of the linked list. 
 * @param {number} val
 * @return {void}
 */
MyLinkedList.prototype.addAtTail = function(val) {
    const node = new LinkNode(val, null);
    this._size++;
    if(this._tail) {
        this._tail.next = node;
        this._tail = node;
        return;
    }
    this._tail = node;
    this._head = node;
};

/**
 * Add a node of value val before the index-th node in the linked list. If index equals to the length of linked list, the node will be appended to the end of linked list. If index is greater than the length, the node will not be inserted. 
 * @param {number} index 
 * @param {number} val
 * @return {void}
 */
MyLinkedList.prototype.addAtIndex = function(index, val) {
    if(index > this._size) return;
    if(index <= 0) {
        this.addAtHead(val);
        return;
    }
    if(index === this._size) {
        this.addAtTail(val);
        return;
    }
    // 获取目标节点的上一个的节点
    const node = this.getNode(index - 1);
    node.next = new LinkNode(val, node.next);
    this._size++;
};

/**
 * Delete the index-th node in the linked list, if the index is valid. 
 * @param {number} index
 * @return {void}
 */
MyLinkedList.prototype.deleteAtIndex = function(index) {
    if(index < 0 || index >= this._size) return;
    if(index === 0) {
        this._head = this._head.next;
        // 如果删除的这个节点同时是尾节点，要处理尾节点
        if(index === this._size - 1){
            this._tail = this._head
        }
        this._size--;
        return;
    }
    // 获取目标节点的上一个的节点
    const node = this.getNode(index - 1);    
    node.next = node.next.next;
    // 处理尾节点
    if(index === this._size - 1) {
        this._tail = node;
    }
    this._size--;
};

// MyLinkedList.prototype.out = function() {
//     let cur = this._head;
//     const res = [];
//     while(cur) {
//         res.push(cur.val);
//         cur = cur.next;
//     }
// };
/**
 * Your MyLinkedList object will be instantiated and called as such:
 * var obj = new MyLinkedList()
 * var param_1 = obj.get(index)
 * obj.addAtHead(val)
 * obj.addAtTail(val)
 * obj.addAtIndex(index,val)
 * obj.deleteAtIndex(index)
 */```

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