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class Solution {
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  public int[] plusOne(int[] digits) {
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    int len = digits.length;
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    // last digit not a 9, just add 1 to it
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    if (digits[len - 1] != 9) {
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      digits[len - 1] = digits[len - 1] + 1;
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      return digits;
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    }
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    // last digit is a 9, find the closest digit that is not a 9
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    else {
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      int i = len - 1;
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      while (i >= 0 && digits[i] == 9) {
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        digits[i] = 0;
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        i--;
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      }
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      if (i == -1) {
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        int[] ret = new int[len + 1];
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        for (int j = 0; j < len; j++) {
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          ret[j + 1] = digits[j];
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        }
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        ret[0] = 1;
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        return ret;
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      }
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      digits[i] = digits[i] + 1;
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    }
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    return digits;
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  }
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}