代码随想录训练营第四天 | 两两交换链表中的节点、删除链表的倒数第N个节点、链表相交、环形链表2两两交换链表中的节点题

两两交换链表中的节点

思路：定义虚拟头节点，定义2个中间temp变量保存next值，不断遍历交换节点

var swapPairs = function(head) {
    const ret = new ListNode(0, head)
    let pre = ret, temp1 = null, temp2 = null
    while(pre.next && pre.next.next) {
        temp1 = pre.next
        temp2 = pre.next.next.next
        pre.next = pre.next.next
        pre.next.next = temp1
        pre.next.next.next = temp2
        pre = temp1
    }

    return ret.next
};

删除链表的倒数第N个节点

题目链接：删除链表的倒数第N个节点

思路：倒数第n个节点已知，先定义虚拟表头，利用快慢指针，让fast先走n + 1步，然后fast & slow一起遍历，当fast指向null的时候，slow的下一个next为需要删除的节点

var removeNthFromEnd = function(head, n) {
    const ret = new ListNode(0, head)
    let fast = slow = ret
    fast = fast.next
    while(n--) {
        fast = fast.next
    }
    while(fast) {
        fast = fast.next
        slow = slow.next
    }

    slow.next = slow.next.next
    return ret.next
};

链表相交

题目链接：链表相交

思路：先获取2个链表的长度，然后相减得到len，先让headA先走len，然后再一起遍历

var getIntersectionNode = function(headA, headB) {
    const getLen = (head) => {
        let len = 0, cur = head
        while(cur) {
            len++
            cur = cur.next
        }
        return len
    }

    let lenA = getLen(headA), lenB = getLen(headB)
    if(lenA < lenB) {
        [lenA, lenB] = [lenB, lenA];
        [headA, headB] = [headB, headA]
    }

    let curA = headA, curB = headB
    let len = lenA - lenB
    while(len--) {
        curA = curA.next
    }

    while(curA) {
        if(curA === curB) {
            return curA
        }
        curA = curA.next
        curB = curB.next
    }

    return null
};

环形链表2

题目链接：环形链表2

思路：先找出fast & slow指针相遇节点，然后遍历

var detectCycle = function(head) {
    if(!head || !head.next) return null
    let slow = head.next, fast = head.next.next
    while(fast && fast.next && fast !== slow) {
        slow = slow.next
        fast = fast.next.next
    }
    if(!fast || !fast.next) return null
    slow = head
    while(fast !== slow) {
        slow = slow.next
        fast = fast.next
    }

    return slow
};