leetcode | Valid Palindrome

Valid Palindrome : https://leetcode.com/problems/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.

解析

alphanumeric characters ：字母或数字型字符

本题难点在于：
1. 全空格字符串应判断为true
2. 字母不区分大小写

一种算法就是，将原字符串剔除所有非字母数字字符，插入到一个新的字符串空间，然后基于新字符串，用2个指针法做判断。算法实现简单，不需额外考虑全空格字符串，但是占用额外空间。

另一种算法是，用2个指针指向字符串两头，当遇到非字母数字字符时跳过，然后做处理。对于全空格字符串，通过判断左指针是否会到达字符串尾部。，优点是不占用额外空间，时间复杂度上也优于第一种（只需遍历一次数组）。

算法 1 实现

class Solution {
public:
    bool isPalindrome(string s) {
        string s2;
        int i = -1;
        while (++i < s.size()) {
            if ((s[i] < ‘A‘ || s[i] > ‘Z‘) && (s[i] < ‘a‘ || s[i] > ‘z‘) && (s[i] < ‘0‘ || s[i] > ‘9‘))
                continue;
            else
                s2 += s[i];
        }
        i = -1;
        int j = s2.size();
        if (j == 0)
            return true;
        int interval = ‘a‘ - ‘A‘;
        while (++i <= --j) {
            if (!(s2[i] == s2[j] || s2[i]+interval == s2[j] || s2[i]-interval == s2[j]))
                return false;
        }
        return true;
    }
};

算法 2 实现

class Solution {
public:
    bool isPalindrome(string s) {
        if (s.size() == 0)
            return true;
        int i = -1;
        int j = s.size();
        int interval = ‘a‘ - ‘A‘;
        while (++i <= --j) {
            while ((i < j) && (s[i] < ‘A‘ || s[i] > ‘Z‘) && (s[i] < ‘a‘ || s[i] > ‘z‘) && (s[i] < ‘0‘ || s[i] > ‘9‘))
                i++;
            while ((i < j) && (s[j] < ‘A‘ || s[j] > ‘Z‘) && (s[j] < ‘a‘ || s[j] > ‘z‘) && (s[j] < ‘0‘ || s[j] > ‘9‘))
                j--;
            // 全空格字符串
            if (i == s.size()-1)
                return true;
            if (s[i] != s[j] && s[i]+interval != s[j] && s[i]-interval != s[j])
                return false;
        }
        return true;
    }
};