leetcode 393. UTF-8 Validation UTF-8编码判断 + 位运算

Dear 丶 2022-06-04 05:41 134阅读 0赞

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

For 1-byte character, the first bit is a 0, followed by its unicode code.  
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.

这道题就是对UTF-8的编码做一个判断，看起来很复杂，其实很简单，只要遍历一次即可，这个用到了位运算，右移判断即可

**需要注意的地方时位运算要注意添加括号**

代码如下：

#include <iostream>
    #include <vector>
    #include <map>
    #include <set>
    #include <queue>
    #include <stack>
    #include <string>
    #include <climits>
    #include <algorithm>
    #include <sstream>
    
    using namespace std;
    
    
    /*
     0xxxxxxx
     110xxxxx 10xxxxxx
     1110xxxx 10xxxxxx 10xxxxxx
     11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
    */
    class Solution
    {
    public:
        bool validUtf8(vector<int>& data) 
        {
            int count = 0;
            for (int one : data)
            {
                if (count == 0)
                {
                    if ((one >> 3) == 0b11110)
                        count = 3;
                    else if ((one >> 4) == 0b1110)
                        count = 2;
                    else if ((one >> 5) == 0b110)
                        count = 1;
                    else if ((one >> 7) == 0b0)
                        count = 0;
                    else
                        return false;
                }
                else
                {
                    if ((one >> 6) == 0b10)
                        count--;
                    else
                        return false;
                }
            }
            return count == 0 ? true : false;
        }
    };