[leetcode] 393. UTF-8 Validation 解題報告

題目鏈接：https://leetcode.com/problems/utf-8-validation/

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.

思路：題目並不難，但卻很難寫的好看．藉助一個計數器來標誌當前是否是開始，或者處在一個多於一個bite的unicode的第幾個．

代碼如下：

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

參考：https://discuss.leetcode.com/topic/57195/concise-c-implementation