LeetCode UTF-8 Validation
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.
给定一个数组，问这个数组是否能表示正确的UTF8字符。UTF8格式在题目中有详细的说明，字节数为1-4个字节，1字节的UTF8第一个bit为0，n字节的UTF8，前n-bits为1，后面跟一个0bit，后面n-1字节前两个bit为10。
解法为我们依次取出字节的前1,3,4,5bits，看是否是1-4字节的UTF8，检查完第一个字节后，根据字节数检查后面的n-1字节。
注意UTF8字节数只可能是1-4，如果发现第一个字节的前5个及以上的bit都是1，直接返回false。代码如下：
[cpp]
class Solution {
public:
bool validUtf8(vector<int>& data) {
vector<int> mask1 = { 0x80,0xe0,0xf0,0xf8 };
vector<int> first = { 0x0,0xc0,0xe0,0xf0 };
int mask2 = 0xc0, second = 0x80;
int i = 0, j = 0;
while (i < data.size()) {
for (j = 0; j < 4; ++j) {
if ((data[i] & mask1[j]) == first[j]) {
while (j–) {
if (++i >= data.size())return false;
if ((data[i] & mask2) != second)return false;
}
break;
}
}
if (j >= 4)return false;
++i;
}
return true;
}
};
[/cpp]
注意位运算的优先级小于判等运算，所以一定记得加括号，要不然WA。。。
本代码提交AC，用时16MS。]]>
