Javascript 实现链表的常见操作

1. 单链表反转

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var reverseList = function (head) {
          
   
    if (!head || !head.next) {
          
   
        return head;
    }
    let preNode = null;
    let currentNode = head;
    let nextNode = currentNode.next;
    while (currentNode.next) {
          
   
        currentNode.next = preNode;
        preNode = currentNode;
        currentNode = nextNode;
        nextNode = currentNode.next;
    }
    // 此时，currentNode 为最后一个节点
    currentNode.next = preNode;
    return currentNode;
};

2. 链表中环的检测

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */

/**
 * @param {ListNode} head
 * @return {boolean}
 */
var hasCycle = function (head) {
          
   
    let slowP = head;
    let fastP = head;
    if (!head || !head.next) {
          
   
        return false;
    }
    // 链表中如果存在环，则两个指针早晚会相遇的！
    while (fastP.next && fastP.next.next) {
          
   
        slowP = slowP.next;
        fastP = fastP.next.next;
        if (slowP === fastP) {
          
   
            return true;
        }
    }
    return false;
};

3. 两个有序的链表合并

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var mergeTwoLists = function (l1, l2) {
          
   
    if (!l1) {
          
   
        return l2;
    }
    if (!l2) {
          
   
        return l1;
    }
    let result = new ListNode();
    let temp = result;
    while (l1 && l2) {
          
   
        if (l1.val <= l2.val) {
          
   
            temp.next = l1;
            l1 = l1.next;
        } else {
          
   
            temp.next = l2;
            l2 = l2.next;
        }
        temp = temp.next;
    }
    if (l1) {
          
   
        temp.next = l1;
    }
    if (l2) {
          
   
        temp.next = l2;
    }
    return result.next;
};

4. 删除链表倒数第 n 个结点

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @param {number} n
 * @return {ListNode}
 */
var removeNthFromEnd = function (head, n) {
          
   
    let preHead = new ListNode();
    preHead.next = head;
    let slowP = preHead;
    let fastP = preHead;
    let i = 0;

    if (!head) {
          
   
        return head;
    }
    
    while (fastP.next) {
          
   
        // 在 fastP 已经走了 n 步以后，slowP 才开始向前走
        if (i >= n) {
          
   
            slowP = slowP.next;
        }
        i++;
        fastP = fastP.next;
    }
    slowP.next = slowP.next.next;
    return preHead.next;
};

5. 求链表的中间结点

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var middleNode = function (head) {
          
   
    let slowP = head;
    let fastP = head;
    if(!head){
          
   
        return head;
    }
    while (fastP && fastP.next) {
          
   
        slowP = slowP.next;
        fastP = fastP.next.next;
    }
    return slowP;
};