393_UTF-8_Validation

UTF-8 Validation

Difficulty Medium

tags bit mask

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.

his 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.

注意：

这里使用了 0b110 这样的写法， 这个在GCC(>4.4)和C++14和VS2015中开始被支持。 参见：Can I use a binary literal in C or C++?

失误的Case：

1. 读题不细致， UTF-8字符最多只有四个字节长， 对于超过四个字节的要判错。

2. 最后的count仍可能大于0，即最后一个UTF8字符并没有给完全， 这种情况要判错。

solution 1

class Solution {
public:
    bool validUtf8(vector<int>& data) {
        int count = 0;
        for (auto c : data) {
            if (count == 0) {
                if ((c >> 5) == 0b110) count = 1;
                else if ((c >> 4) == 0b1110) count = 2;
                else if ((c >> 3) == 0b11110) count = 3;
                else if ((c >> 7)) return false;
            } else {
                if ((c >> 6) != 0b10) return false;
                count--;
            }
        }
        return count == 0;
    }
};