链表 第一天

21.合并两个有序链表

链表结构

    this.val = (val === undefined ? 0 : val)
    this.next = (next === undefined ? null : next)
}

//递归
var mergeTwoLists = function (l1, l2) {
    if(l1===null){
        return l2
    }
    else if(l2===null){
        return l1
    }
    else if(l1.val<l2.val){
        l1.next=mergeTwoLists(l1.next,l2)
        return l1
    }else{
        l2.next=mergeTwoLists(l1,l2.next)
        return l2
    }
};

//迭代

var mergeTwoLists = function (l1, l2) {
    let res = new ListNode()
    let l3=res
    while (l1 && l2) {
        if (l1.val < l2.val) {
            l3.next=l1
            l1 = l1.next
        } else {
            l3.next=l2
            l2 = l2.next
        }
        l3=l3.next
    }
    l3.next=(l1?l1:l2);
    return res.next
};

19.删除链表的倒数底n个节点

function ListNode(val, next) {
    this.val = (val === undefined ? 0 : val)
    this.next = (next === undefined ? null : next)
}

//双指针，快慢指针

var removeNthFromEnd = function (head, n) {
   let fast=head,
       sort=head
    while(n-->0){
        fast=fast.next
    }
    if(fast==null) return head.next
    while(fast.next){
        fast=fast.next
        sort=sort.next
    }
    sort.next=sort.next.next
    return head
};

//递归

let con = 0
var removeNthFromEnd = function (head, n) {
    if (head == null) return null
    head.next = removeNthFromEnd(head.next, n)
    con++
    if (n == con) return head.next;
    return head;
};

24.交换链表的相邻节点

var swapPairs = function(head) {
    let res= new ListNode()
    res.next=head
    let t=res
    
    while(t.next&&t.next.next){
        let l=t.next
            r=t.next.next
        t.next=r
        l.next=r.next
        r.next=l
        t=l
    }
    return res.next
};