[Swift]LeetCode393. UTF-8 编码验证 | UTF-8 Validation

原文地址：https://www.cnblogs.com/strengthen/p/10299741.html 

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.

---

UTF-8 中的一个字符可能的长度为 1 到 4 字节，遵循以下的规则：

1. 对于 1 字节的字符，字节的第一位设为0，后面7位为这个符号的unicode码。
2. 对于 n 字节的字符 (n > 1)，第一个字节的前 n 位都设为1，第 n+1 位设为0，后面字节的前两位一律设为10。剩下的没有提及的二进制位，全部为这个符号的unicode码。

这是 UTF-8 编码的工作方式：

   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

给定一个表示数据的整数数组，返回它是否为有效的 utf-8 编码。

注意:
输入是整数数组。只有每个整数的最低 8 个有效位用来存储数据。这意味着每个整数只表示 1 字节的数据。

示例 1:

data = [197, 130, 1], 表示 8 位的序列: 11000101 10000010 00000001.

返回 true 。
这是有效的 utf-8 编码，为一个2字节字符，跟着一个1字节字符。

示例 2:

data = [235, 140, 4], 表示 8 位的序列: 11101011 10001100 00000100.

返回 false 。
前 3 位都是 1 ，第 4 位为 0 表示它是一个3字节字符。
下一个字节是开头为 10 的延续字节，这是正确的。
但第二个延续字节不以 10 开头，所以是不符合规则的。

---

104ms

class Solution {
    func validUtf8(_ data: [Int]) -> Bool {
        var cnt:Int = 0
        for d in data
        {
            if cnt == 0
            {
                if (d >> 5) == 0b110 {cnt = 1}
                else if (d >> 4) == 0b1110 {cnt = 2}
                else if (d >> 3) == 0b11110 {cnt = 3}
                else if d >> 7 == 1 {return false}
                //else if d>>7 == 0b1 { return false }
            }
            else
            {
                if (d >> 6) != 0b10 {return false}
                cnt -= 1
            }
        }
        return cnt == 0
    }
}