LeetCode 0393. UTF-8 Validation Solution in Java, C++, Python & More | Explanation + Code

Description

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 a 1-byte character, the first bit is a 0, followed by its Unicode code.
2. For an n-bytes character, the first n bits are all one's, the n + 1 bit is 0, followed by n - 1 bytes with the most significant 2 bits being 10.

This is how the UTF-8 encoding would work:

     Number of Bytes   |        UTF-8 Octet Sequence
                       |              (binary)
   --------------------+-----------------------------------------
            1          |   0xxxxxxx
            2          |   110xxxxx 10xxxxxx
            3          |   1110xxxx 10xxxxxx 10xxxxxx
            4          |   11110xxx 10xxxxxx 10xxxxxx 10xxxxxx

x denotes a bit in the binary form of a byte that may be either 0 or 1.

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:

Input: data = [197,130,1]
Output: true
Explanation: data represents the octet sequence: 11000101 10000010 00000001.
It is a valid utf-8 encoding for a 2-bytes character followed by a 1-byte character.

Example 2:

Input: data = [235,140,4]
Output: false
Explanation: data represented the octet sequence: 11101011 10001100 00000100.
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.

 

Constraints:

• 1 <= data.length <= 2 * 104
• 0 <= data[i] <= 255

Solutions

Solution 1: Single Pass

We use a variable cnt to record the current number of bytes that need to be filled starting with 10, initially cnt = 0.

For each integer v in the array:

• If cnt > 0, then check if v starts with 10. If not, return false, otherwise decrement cnt.
• If the highest bit of v is 0, then cnt = 0.
• If the highest two bits of v are 110, then cnt = 1.
• If the highest three bits of v are 1110, then cnt = 2.
• If the highest four bits of v are 11110, then cnt = 3.
• Otherwise, return false.

Finally, if cnt = 0, return true, otherwise return false.

The time complexity is O(n), where n is the length of the array data. The space complexity is O(1).

PythonJavaC++GoTypeScript

class Solution: def validUtf8(self, data: List[int]) -> bool: cnt = 0 for v in data: if cnt > 0: if v >> 6 != 0b10: return False cnt -= 1 elif v >> 7 == 0: cnt = 0 elif v >> 5 == 0b110: cnt = 1 elif v >> 4 == 0b1110: cnt = 2 elif v >> 3 == 0b11110: cnt = 3 else: return False return cnt == 0(code-box)

class Solution { public boolean validUtf8(int[] data) { int cnt = 0; for (int v : data) { if (cnt > 0) { if (v >> 6 != 0b10) { return false; } --cnt; } else if (v >> 7 == 0) { cnt = 0; } else if (v >> 5 == 0b110) { cnt = 1; } else if (v >> 4 == 0b1110) { cnt = 2; } else if (v >> 3 == 0b11110) { cnt = 3; } else { return false; } } return cnt == 0; } }(code-box)

class Solution { public: bool validUtf8(vector<int>& data) { int cnt = 0; for (int& v : data) { if (cnt > 0) { if (v >> 6 != 0b10) { return false; } --cnt; } else if (v >> 7 == 0) { cnt = 0; } else if (v >> 5 == 0b110) { cnt = 1; } else if (v >> 4 == 0b1110) { cnt = 2; } else if (v >> 3 == 0b11110) { cnt = 3; } else { return false; } } return cnt == 0; } };(code-box)

func validUtf8(data []int) bool { cnt := 0 for _, v := range data { if cnt > 0 { if v>>6 != 0b10 { return false } cnt-- } else if v>>7 == 0 { cnt = 0 } else if v>>5 == 0b110 { cnt = 1 } else if v>>4 == 0b1110 { cnt = 2 } else if v>>3 == 0b11110 { cnt = 3 } else { return false } } return cnt == 0 }(code-box)

function validUtf8(data: number[]): boolean { let cnt = 0; for (const v of data) { if (cnt > 0) { if (v >> 6 !== 0b10) { return false; } --cnt; } else if (v >> 7 === 0) { cnt = 0; } else if (v >> 5 === 0b110) { cnt = 1; } else if (v >> 4 === 0b1110) { cnt = 2; } else if (v >> 3 === 0b11110) { cnt = 3; } else { return false; } } return cnt === 0; }(code-box)