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Question

I\'m trying to solve a question and my question here is why doesn\'t my solution work?. Here\'s the question and below\'s the answer.

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Answer 1

Java solution with space and time complexity O(n)

public int numDecodings(String s) {
    int n = s.length();
    if (n > 0 && s.charAt(0) == '0')
        return 0;
    int[] d = new int[n + 1];
    d[0] = 1;
    d[1] = s.charAt(0) != '0' ? 1 : 0;
    for (int i = 2; i <= n; i++) {
        if (s.charAt(i - 1) > '0')
            d[i] = d[i] + d[i - 1];

        if (s.charAt(i - 2) == '2' && s.charAt(i - 1) < '7')
            d[i] = d[i - 2] + d[i];
        if (s.charAt(i - 2) == '1' && s.charAt(i - 1) <= '9')
            d[i] = d[i - 2] + d[i];
    }
    return d[n];
}