Leetcode 393 - UTF-8 Validation

Note:

  • The description was so confusing.
  • Just enumerate all situations and put them in a dfs.
  • Use memo coz there might be repetitive calculations.
  • Use e.toString(2) to convert decimals to binariy strs. (Reversed operation is to use parseInt(str, 2)).

Question:

Given an integer array data representing the data, return whether it is a valid UTF-8 encoding.

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:

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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

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:

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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.

Code:

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/**
* @param {number[]} data
* @return {boolean}
*/
var validUtf8 = function(data) {
let memo = {};
data = data.map(e => {
let binaryStr = e.toString(2);
if (binaryStr.length > 8) {
binaryStr = binaryStr.slice(0, 8);
return binaryStr;
} else if (binaryStr.length < 8) {
binaryStr = '0'.repeat(8 - binaryStr.length) + binaryStr;
return binaryStr;
}
return binaryStr;
});
return dfs(data, 0);

function dfs(data, startIndx) {
if (startIndx >= data.length) return true;
if (memo[startIndx] !== undefined) return memo[startIndx];
if (data[startIndx][0] === '0') {
const bool = dfs(data, startIndx + 1);
memo[startIndx] = bool;
return bool;
}
if (data[startIndx].slice(0, 3) === '110' && data[startIndx + 1] && data[startIndx + 1].slice(0, 2) === '10') {
const bool = dfs(data, startIndx + 2);
memo[startIndx] = bool;
return bool;
}
if (data[startIndx].slice(0, 4) === '1110' && data[startIndx + 1] && data[startIndx + 1].slice(0, 2) === '10' && data[startIndx + 2] && data[startIndx + 2].slice(0, 2) === '10') {
const bool = dfs(data, startIndx + 3);
memo[startIndx] = bool;
return bool;
}
if (data[startIndx].slice(0, 5) === '11110' && data[startIndx + 1] && data[startIndx + 1].slice(0, 2) === '10' && data[startIndx + 2] && data[startIndx + 2].slice(0, 2) === '10' && data[startIndx + 3] && data[startIndx + 3].slice(0, 2) === '10') {
const bool = dfs(data, startIndx + 4);
memo[startIndx] = bool;
return bool;
}
memo[startIndx] = false;
return false;
}
};