# Question

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
--------------------+---------------------------------------------
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

Be careful of operator precedence in Java. == or != has higher precedence over &.

class Solution {
public boolean validUtf8(int[] data) {
int i = 0;
int bytes = 0;
while (i < data.length) {
if(bytes > 0) { // remaining bytes
return false;
else bytes--;
} else if(data[i] < 128) { // one byte
} else if ((data[i] & FOUR_BYTES_MASK) == FOUR_BYTES_HEAD) { // 1st byte in 4 bytes
bytes = 3;
} else if ((data[i] & THREE_BYTES_MASK) == THREE_BYTES_HEAD) { // 1st byte in 3 bytes
bytes = 2;
} else if ((data[i] & TWO_BYTES_MASK) == TWO_BYTES_HEAD) { // 1st byte in 2 bytes
bytes = 1;
} else {
return false;
}
i++;
}
return bytes == 0;
}
}