206. Reverse Linked List

206. Reverse Linked List

Reverse a singly linked list.

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Hint: A linked list can be reversed either iteratively or recursively. Could you implement both?

iterative 解法：

总结就是得到下一个节点，更改当前节点指向，将指针往下移动，直到过完整个linkedlist.

/** * Definition for singly-linked list. * public class ListNode { * int val; * ListNode next; * ListNode(int x) { val = x; } * } */class Solution { public ListNode reverseList(ListNode head) { ListNode prev = null; ListNode cur = head; while(cur!=null){ ListNode next = cur.next; cur.next = prev; prev = cur; cur = next; } return prev; }}

/** * Definition for singly-linked list. * public class ListNode { * int val; * ListNode next; * ListNode(int x) { val = x; } * } */class Solution { public ListNode reverseList(ListNode head) { if(head == null || head.next==null) return head; // 1->2->3->4->null ListNode nextNode = head.next; // 2->3->4->null ListNode reversed = reverseList(nextNode); //4->3->2->null nextNode.next = head; //4->3->2->1 head.next = null; //4->3->2->1->null return reversed; }}

/** * Definition for singly-linked list. * public class ListNode { * int val; * ListNode next; * ListNode(int x) { val = x; } * } */class Solution { public ListNode reverseList(ListNode head) { ListNode rev = null; ListNode temp1 = head; while(temp1 != null) { if (rev == null) { rev = new ListNode(temp1.val); } else { ListNode tnew = new ListNode(temp1.val); tnew.next = rev; rev = tnew; } temp1 = temp1.next; } return rev; }}

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