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Find Missing Sequence

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Give an array A of n integers where 1 <= a[i] <= K.
Find out the length of the shortest sequence that can be constructed out of numbers 1, 2, .. k that is NOT a subsequence of A.
eg. A = [4, 2, 1, 2, 3, 3, 2, 4, 1], K = 4
All single digits appears.
Each of the 16 double digit sequences, (1,1), (1, 2), (1, 3), (1, 4), (2, 1), (2, 2) ... appears.
Because (1, 1, 2) doesn't appear, return 3.
Solution:
The optimal solution is a little tricky. Can you prove it?

public int shortestNonSeq(int[] a, int k) {
        int result = 0;
        Set flag = new HashSet();
        for (int i = 0; i < a.length; i  ) {
            if (!flag.contains(a[i])) {
                flag.add(a[i]);
                if (flag.size() == k) {
                    result  ;
                    flag = new HashSet<>();
                }
            }
        }
        return result   1;
    }


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