Today I have done leetcode's September LeetCoding Challenge with cpp.

September LeetCoding Challenge 13

Description

UTF-8 Validation

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 * 10^4
• 0 <= data[i] <= 255

Solution

auto speedup = [](){
  cin.tie(nullptr);
  cout.tie(nullptr);
  ios::sync_with_stdio(false);
  return 0;
}();
class Solution {
public:
  bool validUtf8(vector<int>& data) {
    int pos = 0;
    int len = data.size();
    while(pos < len) {
      int curPos = pos;
      if(!(data[pos] & 0b10000000)) {
        pos += 1;
      } else {
        for(int i = 2; i < 5; ++i) {
          int mask = (1 << i) - 1;
          // not have first n's 1 or not have (n + 1)'s bit as 0
          if(((data[pos] >> (8 - i)) & mask) != mask || ((data[pos] >> (7 - i)) & 1)) continue;
          if(pos > len - i) return false;
          for(int j = 1; j < i; ++j) {
            cout << i << ' ' << data[pos + j] << endl;
            if((data[pos + j] & 0b11000000) != 0b10000000) return false;
          }
          pos += i;
          break;
        }
      }
      if(pos == curPos) return false;
    }
    return true;
  }
};

// Accepted
// 49/49 cases passed (8 ms)
// Your runtime beats 92.07 % of cpp submissions
// Your memory usage beats 7.65 % of cpp submissions (14.1 MB)