2014
11-18

# LeetCode-Valid Palindrome[字符串]

### Valid Palindrome

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.

// Leet Code, Valid Palindrome
// 时间复杂度O(n)，空间复杂度O(1)
class Solution {
public:
bool isPalindrome(string s) {
transform(s.begin(), s.end(), s.begin(), ::tolower);
auto left = s.begin(), right = prev(s.end());
while (left < right) {
if (!::isalnum(*left))  ++left;
else if (!::isalnum(*right)) --right;
else if (*left != *right) return false;
else{ left++, right--; }
}
return true;
}
};


Java代码:

class Solution:
# @param s, a string
# @return a boolean
def isPalindrome(self, s):
low = 0
high = len(s)-1
s = s.lower()
while low < high:
while s[low].isalnum() is False and low < high: low += 1
while s[high].isalnum() is False and low < high: high -= 1
# print(s[low],"  ", s[high])
if s[low] != s[high]:
return False
low += 1
high -= 1
return True