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.

Solution

public class Solution {
    public boolean validUtf8(int[] data) {
        if(data == null || data.length == 0) {
            return false;
        }

        int idx = 0;
        boolean header = true;
        int con = 0;
        int supposeLen = 0;
        while(idx < data.length) {
            if(header) {
                supposeLen = 0;
                for(int i = 7; i >= 0; i--) {
                    if(testBit(data[idx], i)) {
                        supposeLen++;
                    } else {
                        break;
                    }
                }
                if(supposeLen == 0) {
                    idx++;
                    continue;
                }
                if(supposeLen == 1 || supposeLen > 4) return false;

                int remain = data.length - idx - 1;
                if(remain < (supposeLen - 1)) {
                    return false;
                }
                header = false;
                con = 0;
            } else {
                if(follow(data[idx])) {
                    con++;
                    if(con == supposeLen - 1) {
                        header = true;
                        con = 0;
                    }
                } else {
                    return false;
                }
            }
            idx ++;
        }
        return true;
    }

    public boolean testBit(int num, int pos) {
        return (num & (1 << pos)) > 0;
    }

    public boolean follow(int num) {
        return testBit(num, 7) && !testBit(num, 6);
    }
}