力扣141题、142题、83题、167题（快慢指针,左右指针）

141、环形链表

基本思想：

使用快慢指针，如果是环形的，快指针一定会追上慢指针

具体实现：

循环让快慢指针往后跑，

循环条件是快指针不等于慢指针，

快指针等于慢指针则跳出循环

如果在循环内，快指针指向了null，

跳出循环返回不是环形链表

代码：

class Solution:
    def hasCycle(self, head: ListNode) -> bool:
        if not head or not head.next:
            return False
        
        slow = head
        fast = head.next

        while slow != fast:
            if not fast or not fast.next:
                return False
            slow = slow.next
            fast = fast.next.next
        
        return True

class Solution:
    def detectCycle(self, head: ListNode) -> ListNode:
        fast, slow = head, head
        while True:
            if not (fast and fast.next): return
            fast, slow = fast.next.next, slow.next
            if fast == slow: break
        fast = head
        while fast != slow:
            fast, slow = fast.next, slow.next
        return fast

public class Solution {
    public boolean hasCycle(ListNode head) {
        ListNode fast, slow;
        fast = slow = head;
        while (fast != null && fast.next != null){
            fast = fast.next.next;
            slow = slow.next;
            if (fast == slow) return true;
        }
        return false;
    }
}

142、环形链表II

基本思想：

快慢指针

具体实现：

1、快慢指针相遇时，慢指针走了k步，快指针走了2k步

2、设相遇点到环起点距离为m

3、k-m是头结点到环起点的距离

从相遇点到环起点的距离也是k-m

4、让两个指针的速度统一，都一步一个节点，

一个指针从头结点走，走到环起点走了k-m步

一个指针从相遇点走，走到环起点走了k-m步

两个指针这样相遇则是环起点

代码：

class Solution:
    def detectCycle(self, head: ListNode) -> ListNode:
        fast, slow = head, head
        while True:
            if not (fast and fast.next):
                return
            fast, slow = fast.next.next, slow.next
            if fast == slow: break
        fast = head
        while fast != slow:
            fast, slow = fast.next, slow.next
        return fast

public class Solution {
    public ListNode detectCycle(ListNode head) {
        ListNode fast, slow;
        fast = slow = head;
        while (fast != null && fast.next != null){
            fast = fast.next.next;
            slow = slow.next;
            if (fast == slow) {
                fast = head;
                while (fast != slow){
                    fast = fast.next;
                    slow = slow.next;
                }
            return fast;
            }
        }
        return null;
    }
}

167、两数之和II

基本思想：

左、右指针

具体实现：

类似于二分搜索

代码：

class Solution:
    def twoSum(self, numbers: List[int], target: int) -> List[int]:
        left = 0
        right = len(numbers)-1
        res = []
        while left < right:
            twosum = numbers[left] + numbers[right]
            if twosum == target:
                res.append(left+1)
                res.append(right+1)
                return res
            elif twosum < target:
                left += 1
            else:
                right -= 1
        return -1

83、删除排序链表中的重复元素

具体实现：

慢指针走在后面，快指针走在前面探路，找到一个不重复的元素就让慢指针指向快指针，指针快指针遍历完整个链表

代码：

class Solution:
    def deleteDuplicates(self, head: ListNode) -> ListNode:
        if not head:
            return head

        slow = head
        fast = slow.next
        while fast:
            if slow.val != fast.val:
                slow.next = fast
                slow = slow.next
            fast = fast.next

        slow.next = None 
        return head