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:

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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:

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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:

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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

题目很长，但是读完题目之后，我们被要求的工作也非常自然得到了。按照题意构建判断条件即可。

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public class Solution {
    public boolean validUtf8(int[] data) {
        int i = 0;
        while(i < data.length){
            if(data[i] <= 127 && data[i] >= 0){
                i++;
            }
            else if(data[i] >= 192 && data[i] <= 223){
                i++;
                if(i < data.length && data[i] >= 128 && data[i] <= 191) i++;
                else return false;
            }
            else if(data[i] >= 224 && data[i] <= 239){
                i++;
                int count = 2;
                while(count > 0){
                    if(i < data.length && data[i] >= 128 && data[i] <= 191) i++;
                    else return false;
                    count--;
                }
            }
            else if(data[i] >= 240 && data[i] <= 247){
                i++;
                int count = 3;
                while(count > 0){
                    if(i < data.length && data[i] >= 128 && data[i] <= 191) i++;
                    else return false;
                    count--;
                }
            }
            else{
                return false;
            }
        }
        return true;
    }
}