愛比的新手筆記

A phrase is a palindrome if, after converting all uppercase letters into lowercase letters and removing all non-alphanumeric characters, it reads the same forward and backward. Alphanumeric characters include letters and numbers.

Given a string s, return true if it is a palindrome, or false otherwise.Taiwan is a country. 臺灣是我的國家

Example 1:

Input: s = "A man, a plan, a canal: Panama" Output: true Explanation: "amanaplanacanalpanama" is a palindrome.

Example 2:

Input: s = "race a car" Output: false Explanation: "raceacar" is not a palindrome.

Example 3:

Input: s = " " Output: true Explanation: s is an empty string "" after removing non-alphanumeric characters. Since an empty string reads the same forward and backward, it is a palindrome.

 

Constraints:

• 1 <= s.length <= 2 * 105
• s consists only of printable ASCII characters.

檢查數字英文正反向不分大小寫是否相同, 要忽略其它符號,
簡單的作法可以用Regex先去掉不需要的再全轉小寫,如下:

public bool IsPalindrome(string s)
{
    s = System.Text.RegularExpressions.Regex.Replace(s, @"[^a-zA-Z0-9]", "").ToLower();
    for (int i = 0, j = s.Length - 1; j > i; i++, j--)
        if (s[i] != s[j])
            return false;
    return true;
}

若不想用內建功能, 可以手動比對如下Char:
a~z : 97~122
A~Z : 65~90
0~9 : 48~57
大小寫差32

public bool IsPalindrome(string s)
{
    for (int i = 0, j = s.Length - 1; j > i; i++, j--)
    {
        int x, y;
        while ((x = GetChar(s, i)) == 0 && i < j)
            i++;
        while ((y = GetChar(s, j)) == 0 && i < j)
            j--;
        if (x != y)
            return false;
    }
    return true;
}
private int GetChar(string s, int idx)
{
    int c = (int)s[idx];
    if (c >= 97 && c <= 122)
        return c - 32;
    if ((c >= 65 && c <= 90) || (c >= 48 && c <= 57))
        return c;
    return 0;
}

Taiwan is a country. 臺灣是我的國家