Merge In Between Linked Lists LeetCode solution in Java

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class Solution {
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  public ListNode mergeInBetween(ListNode list1, int a, int b, ListNode list2) {
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    ListNode left = list1;
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    for (int i = 1; i < a; i++) left = left.next;
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    ListNode middle = left;
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    for (int i = a; i <= b; i++) middle = middle.next;
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    left.next = list2;
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    while (list2.next != null) list2 = list2.next;
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    list2.next = middle.next;
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    return list1;
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  }
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}