Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.
For example,"A man, a plan, a canal: Panama" is a palindrome."race a car" is not a palindrome.
Note:
Have you consider that the string might be empty? This is a good question to ask during an interview.
For the purpose of this problem, we define empty string as valid palindrome.
分析：题意为 给出一个字符串，检查它是不是一个回文的情况。判断时只考虑字母数字的字符并且忽略大小写。
注意点 1、考虑字符串为空的情况，并将其作为回文的
2、忽略大小写的差别，比较之前先将大小写转换为一致的
3、非字母数字字符要跳过，将它滤除，得到有效字符串
4、对有效字符串前后相应位置进行比较判断
代码如下：
class Solution {
public:
bool isStr(char &ch){
if(ch >= '0' && ch <= '9'){
return true;
} else if(ch >= 'a' && ch <= 'z'){
return true;
} else if(ch >= 'A' && ch <= 'Z'){
ch += 32;
return true;
}
return false;
}
bool isPalindrome(string s) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
int len = s.length();
if(len == 0){
return true;
}
string str = "";
for(int i = 0; i < len; i++){ // remove illegal char, such as "?" "/" ...
if(isStr(s[i])){
str += s[i];
}
}
len = str.length();
int mid = (len + 1) / 2;
for(int i = 0; i < mid; i++){
if(str[i] != str[len - 1 - i]){ // check front and end char
return false;
}
}
return true;
}
};
其他解法：