思路是保存一个前驱节点 preHead = null
class Solution {
public ListNode reverseList(ListNode head) {
if (head == null) return null;
ListNode reHead = null;
ListNode pre = null;
ListNode tmp = head;
while(tmp != null) {
ListNode next = tmp.next;
if (next == null) reHead = tmp;
tmp.next = pre;
pre = tmp;
tmp = next;
}
return reHead;
}
}










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