[leetcode] 393. UTF-8 Validation

Description

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.      

分析

• 這個解法非常的簡潔，通過平移的方式判斷每個位元組的頭部是否合法，如果不合法就傳回false，合法的話，記錄剩下位元組的頭部是否合法，這裡cnt儲存需要判斷剩下位元組的位元組數。

代碼

class Solution {
public:
    bool validUtf8(vector<int>& data) {
        int cnt=0;
        for(int d: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) return false;
            }else{
                if((d>>6)!=0b10) return false;
                cnt--;
            }
        }
        return cnt==0;
    }
};      

參考文獻