UTF-8 Validation

A character in UTF8 can be from 1 to 4 bytes long, subjected to the following rules:

  1. For 1-byte character, the first bit is a 0, followed by its unicode code.
  2. For n-bytes character, the first n-bits are all one's, the n+1 bit is 0, followed by n-1 bytes with most significant 2 bits being 10.
This is how the UTF-8 encoding would work:

   Char. number range  |        UTF-8 octet sequence
      (hexadecimal)    |              (binary)
   --------------------+---------------------------------------------
   0000 0000-0000 007F | 0xxxxxxx
   0000 0080-0000 07FF | 110xxxxx 10xxxxxx
   0000 0800-0000 FFFF | 1110xxxx 10xxxxxx 10xxxxxx
   0001 0000-0010 FFFF | 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
Given an array of integers representing the data, return whether it is a valid utf-8 encoding.

Note:
The input is an array of integers. Only the least significant 8 bits of each integer is used to store the data. This means each integer represents only 1 byte of data.

Example 1:

data = [197, 130, 1], which represents the octet sequence: 11000101 10000010 00000001.

Return true.
It is a valid utf-8 encoding for a 2-bytes character followed by a 1-byte character.

Example 2:

data = [235, 140, 4], which represented the octet sequence: 11101011 10001100 00000100.

Return false.
The first 3 bits are all one's and the 4th bit is 0 means it is a 3-bytes character.
The next byte is a continuation byte which starts with 10 and that's correct.
But the second continuation byte does not start with 10, so it is invalid.

Solution:

class Solution {
    public boolean validUtf8(int[] data) {
        if (data.length == 0) return true;        
        int[] binary = decimalToBinary(data[0]);
        if (binary[0] == 0) {
            return validUtf8(Arrays.copyOfRange(data, 1, data.length));
        } else {
            int bytes = 0;
            while (bytes <= 3 && binary[bytes] == 1) {
                bytes ++;
            }
            if (bytes < 2 || bytes > 4) {
                return false;
            }
            return helper(Arrays.copyOfRange(data, 0, bytes)) && validUtf8(Arrays.copyOfRange(data, bytes, data.length));
        }
    }
    
    private boolean helper(int[] data) {
        int bytes = data.length;
        for (int i = 0; i < bytes; i ++) {
            int[] binary = decimalToBinary(data[i]);
            if (i == 0) {
                for (int j = 0; j < bytes; j ++) {
                    if (binary[j] != 1) {
                        return false;
                    }
                }
                if (binary[bytes] != 0) {
                    return false;
                }
            } else {
                if (binary[0] != 1) return false;
                if (binary[1] != 0) return false;
            }
        }
        return true;
    }
    
    private int[] decimalToBinary(int number) {
        int[] result = new int[8];
        for (int i = 7; i >= 0; i --) {
            if (number > 0) {
                result[i] = (number & 1);
                number >>= 1;
            }
        }
        return result;
    }
}