Check Array Formation Through Concatenation LeetCode solution in Java

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class Solution {
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  public boolean canFormArray(int[] arr, int[][] pieces) {
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    HashMap<Integer, int[]> hm = new HashMap();
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    for (int[] list : pieces) hm.put(list[0], list);
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    int index = 0;
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    while (index < arr.length) {
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      if (!hm.containsKey(arr[index])) return false;
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      int[] list = hm.get(arr[index]);
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      for (int val : list) {
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        if (index >= arr.length || val != arr[index]) return false;
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        index++;
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      }
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    }
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    return true;
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  }
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}