链表的双指针技巧 leetcode 141、142、160、19、876

1. 在调用 next 字段之前，始终检查节点是否为空。

获取空节点的下一个节点将导致空指针错误。例如，在我们运行 fast = fast.next.next 之前，需要检查 fast 和 fast.next 不为空。

2. 仔细定义循环的结束条件。

141. 环形链表

 

var hasCycle = function(head) {
    if(!head || !head.next) return false;
    let slow = head;
    let fast = head.next;
    while(slow != fast){
        if(!fast || !fast.next) return false;
        fast = fast.next.next;
        slow = slow.next;
    }
    return true;
};

 

142. 环形链表 II

 

 

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

 

160. 相交链表

 

 

var getIntersectionNode = function(headA, headB) {
    let pa = headA, pb = headB;
    while(pa !== pb){
        pa = pa === null?headB:pa.next;
        pb = pb === null?headA:pb.next;
    }
    return pa;
};

 

19. 删除链表的倒数第 N 个结点

 

var removeNthFromEnd = function(head, n) {
    let dummyhead = new ListNode(0,head);
    let fast = head;
    let slow = dummyhead;
    for(let i = 0; i < n; i++){
        fast = fast.next;
    }
    while(fast != null){
        fast = fast.next;
        slow = slow.next;
    }
    slow.next = slow.next.next;
    return dummyhead.next;
};

 

876. 链表的中间结点

var middleNode = function(head) {
    let fast = head;
    let slow = head;
    while(fast != null && fast.next != null){
        fast = fast.next.next;
        slow = slow.next;
    }
    return slow;
};