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/**
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 * Definition for singly-linked list. public class ListNode { int val; ListNode next; ListNode() {}
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 * ListNode(int val) { this.val = val; } ListNode(int val, ListNode next) { this.val = val;
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 * this.next = next; } }
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 */
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class Solution {
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  public ListNode reverseList(ListNode head) {
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    if (head == null || head.next == null) return head;
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    ListNode curr = head;
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    ListNode temp = null, next = curr.next;
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    curr.next = null;
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    while (curr != null && next != null) {
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      // before cutting off link between next & its next, save next.next
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      temp = next.next;
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      // let next point to curr
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      next.next = curr;
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      curr = next;
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      next = temp;
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    }
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    return curr;
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  }
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}