[Swift]LeetCode393. UTF-8 編碼驗證 | UTF-8 Validation

原文地址：http://www.javashuo.com/article/p-vomruado-s.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 中的一個字符可能的長度爲 1 到 4 字節，遵循如下的規則：

1. 對於 1 字節的字符，字節的第一位設爲0，後面7位爲這個符號的unicode碼。
2. 對於 n 字節的字符 (n > 1)，第一個字節的前 n 位都設爲1，第 n+1 位設爲0，後面字節的前兩位一概設爲10。剩下的沒有說起的二進制位，所有爲這個符號的unicode碼。

這是 UTF-8 編碼的工做方式：

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

給定一個表示數據的整數數組，返回它是否爲有效的 utf-8 編碼。

注意:
輸入是整數數組。只有每一個整數的最低 8 個有效位用來存儲數據。這意味着每一個整數只表示 1 字節的數據。

示例 1:

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

返回 true 。
這是有效的 utf-8 編碼，爲一個2字節字符，跟着一個1字節字符。

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 位爲 0 表示它是一個3字節字符。
下一個字節是開頭爲 10 的延續字節，這是正確的。
但第二個延續字節不以 10 開頭，因此是不符合規則的。

---

104ms

 1 class Solution {
 2     func validUtf8(_ data: [Int]) -> Bool {
 3         var cnt:Int = 0
 4         for d in data
 5         {
 6             if cnt == 0
 7             {
 8                 if (d >> 5) == 0b110 {cnt = 1}
 9                 else if (d >> 4) == 0b1110 {cnt = 2}
10                 else if (d >> 3) == 0b11110 {cnt = 3}
11                 else if d >> 7 == 1 {return false}
12                 //else if d>>7 == 0b1 { return false }
13             }
14             else
15             {
16                 if (d >> 6) != 0b10 {return false}
17                 cnt -= 1
18             }
19         }
20         return cnt == 0
21     }
22 }