1分钟 - Linked List Cycle II
2014-05-03 04:27
330 查看
Given a linked list, return the node where the cycle begins. If there is no cycle, return
Follow up:
Can you solve it without using extra space?
/**
* Definition for singly-linked list.
* class ListNode {
* int val;
* ListNode next;
* ListNode(int x) {
* val = x;
* next = null;
* }
* }
*/
public class Solution {
public ListNode detectCycle(ListNode head) {
HashSet<ListNode> nodes = new HashSet<ListNode>();
ListNode current = head;
while (current != null) {
if (nodes.contains(current)) {
return current;
} else {
nodes.add(current);
}
current = current.next;
}
return null;
}
}
Follow up :
已经知道怎么做了就么意思了。随便写写3分钟写出来算了。
/**
* Definition for singly-linked list.
* class ListNode {
* int val;
* ListNode next;
* ListNode(int x) {
* val = x;
* next = null;
* }
* }
*/
public class Solution {
public ListNode detectCycle(ListNode head) {
HashSet<ListNode> nodes = new HashSet<ListNode>();
ListNode slow = head;
ListNode fast = head;
while (fast != null) {
fast = fast.next;
if (fast == null) {
return null;
}
fast = fast.next;
slow = slow.next;
if (slow == fast) {
break;
}
}
if (fast == null) {
return null;
}
fast = head;
while (fast != slow) {
slow = slow.next;
fast = fast.next;
}
return fast;
}
}
null.
Follow up:
Can you solve it without using extra space?
/**
* Definition for singly-linked list.
* class ListNode {
* int val;
* ListNode next;
* ListNode(int x) {
* val = x;
* next = null;
* }
* }
*/
public class Solution {
public ListNode detectCycle(ListNode head) {
HashSet<ListNode> nodes = new HashSet<ListNode>();
ListNode current = head;
while (current != null) {
if (nodes.contains(current)) {
return current;
} else {
nodes.add(current);
}
current = current.next;
}
return null;
}
}
Follow up :
已经知道怎么做了就么意思了。随便写写3分钟写出来算了。
/**
* Definition for singly-linked list.
* class ListNode {
* int val;
* ListNode next;
* ListNode(int x) {
* val = x;
* next = null;
* }
* }
*/
public class Solution {
public ListNode detectCycle(ListNode head) {
HashSet<ListNode> nodes = new HashSet<ListNode>();
ListNode slow = head;
ListNode fast = head;
while (fast != null) {
fast = fast.next;
if (fast == null) {
return null;
}
fast = fast.next;
slow = slow.next;
if (slow == fast) {
break;
}
}
if (fast == null) {
return null;
}
fast = head;
while (fast != slow) {
slow = slow.next;
fast = fast.next;
}
return fast;
}
}
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