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## Problem

Given an integer array data representing the data, return whether it is a valid UTF-8 encoding (i.e. it translates to a sequence of valid UTF-8 encoded characters).

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.


## Code

class Solution {
public boolean validUtf8(int[] data) {
int group = 0;

// 1000 0000
int mask1 = 1 << 7;
// 0100 0000
int mask2 = 1 << 6;

for (int i = 0; i < data.length; i++) {
int curr = data[i];
if (group == 0) {
// 开头有几个1
// 110x xxxx - 1 group
// 1110 xxxx - 2 group
// 1111 0xxx - 3 group
// 1000 0000
int mask = 1 << 7;
while ((mask & curr) != 0) {
group += 1;
}

// 0xxxxxxx 1-byte - valid
if (group == 0) {
continue;
}

if (group > 4 || group == 1) {
return false;
}
} else {
// 后续表示要以10开头
if (!((curr & mask1) != 0 && (mask2 & curr) == 0)) {
return false;
}
}
group -= 1;
}

return group == 0;
}
}