Leetcode: UTF-8 Validation

Leetcode: UTF-8 Validation



Similar Problems:


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.

Github: code.dennyzhang.com

Credits To: leetcode.com

Leave me comments, if you have better ways to solve.


  • Solution:
// Blog link: https://code.dennyzhang.com/utf-8-validation
// Basic Ideas:
// 0xx -> A; 10x -> B; 110x -> C, 1110 -> D, 11110 -> E
// EBBB, DBB, CB, A
//
// Complexity: Time O(n), Space O(n)
func validUtf8(data []int) bool {
    count := 0
    for _, num := range data {
        // 1111 1XXX
        if num & 248 == 248 { return false }
        // 0XXX XXXX
        if num & 128 == 0 { 
            if count != 0 { return false }
            continue 
        }

        expected := 0
        // 1111 0XXX
        if num & 240 == 240 { 
            expected = 3 
        } else {
            // 1110 XXXX
            if num & 224 == 224 {
                expected = 2
            } else {
                // 110X XXXX
                if num & 192 == 192 {
                    expected = 1
                }
            }
        }

        if expected == 0 {
            if count <= 0 { return false }
            count--
        } else {
            if count != 0 { return false }
            count = expected
        }
    }
    return count == 0
}
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