Merge k Sorted Lists LeetCode solution in Java

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class Solution {
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  public ListNode mergeKLists(ListNode[] lists) {
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    if (lists == null || lists.length < 1) return null;
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    // add the first chunk of linkedlist to res,
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    // so later we started from index 1
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    ListNode res = lists[0];
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    // traverse the lists and start merge by calling mergeTwo
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    for (int i = 1; i < lists.length; i++) {
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      res = mergeTwo(res, lists[i]);
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    }
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    return res;
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  }
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  // leetcode 21 technics
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  private ListNode mergeTwo(ListNode l1, ListNode l2) {
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    if (l1 == null) return l2;
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    if (l2 == null) return l1;
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    if (l1.val < l2.val) {
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      l1.next = mergeTwo(l1.next, l2);
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      return l1;
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    } else {
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      l2.next = mergeTwo(l2.next, l1);
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      return l2;
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    }
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  }
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}