393. UTF-8 Validation

A character in UTF8 can be from1 to 4 byteslong, 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.

class Solution {
public:
    bool validUtf8(vector<int>& data) {
        int count = 0;
        for (auto d: data){
            if(count == 0){
                if(d >> 5 == 0b110) count = 1;  // follwing one bytes
                else if(d >> 4 == 0b1110) count = 2;  // follwing two bytes
                else if(d >> 3 == 0b11110) count = 3;   // follwing three bytes 
                else if(d >> 7) return false; // for chars more than 4 bytes, it will be returned here ; for input [255], for 1 byte char: the first bit is 0
            }
            else{
                if (d >> 6 != 0b10) return false;
                count--;
            }
        }

        return count == 0;
    }
};

class Solution(object):
    def validUtf8(self, data):
        """
        :type data: List[int]
        :rtype: bool
        """
        cnt = 0
        for d in data:
            if cnt:
                if d > 191 or d < 128: return False # 0xxxxxxx or 11xxxxxx
                cnt -= 1
            else:
                if d > 247: return False # 11111xxx -> more than 4 bytes
                if d > 239: cnt = 3 #11110xxx
                elif d > 223: cnt = 2 #1110xxxx
                elif d > 191: cnt = 1 #110xxxxx
                elif 0 <= d < 128: continue #0xxxxxxx
                else: return False;

        return not cnt