验证回文串 - 简单

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c++

topc：LCR 018. 验证回文串 - 力扣（LeetCode）

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Sometime really donnot want to write things just because of donot want to. Make excuses like not feel good or have no time. 

Inspect the topic first:

The first question is how to transelate uppercases to lowercases. 

So I refer to some of my frienmds and they told me in c++, a standard usage is here

int tolower(int c);

and if you want to turn the letters to uppercases, that is easy:

int toupper(int c);

why need to transform the letters? because in another case here125. 验证回文串 - 力扣（LeetCode）125. 验证回文串 - 如果在将所有大写字符转换为小写字符、并移除所有非字母数字字符之后，短语正着读和反着读都一样。则可以认为该短语是一个 回文串 。字母和数字都属于字母数字字符。给你一个字符串 s，如果它是 回文串 ，返回 true ；否则，返回 false 。 示例 1：输入: s = "A man, a plan, a canal: Panama"输出：true解释："amanaplanacanalpanama" 是回文串。示例 2：输入：s = "race a car"输出：false解释："raceacar" 不是回文串。示例 3：输入：s = " "输出：true解释：在移除非字母数字字符之后，s 是一个空字符串 "" 。由于空字符串正着反着读都一样，所以是回文串。 提示： * 1 <= s.length <= 2 * 105 * s 仅由可打印的 ASCII 字符组成https://leetcode.cn/problems/valid-palindrome/

need to transform uppercases and lowercases. the rest of them totally same.

Then I need to pointers i and j. pointrt i travels from left to right. pointer j travels from right to left at the same time. check tolower[i] != tolower[j]. If they say yes everytime, then comes true.

class Solution {
public:
    bool isPalindrome(std::string s) {
        int i = 0;
        int j = s.length() - 1;

            // 比较字符，忽略大小写
            if (tolower(s[i]) != tolower(s[j])) {
                return false;
            i++;
            j--;
        }
        
        return true;
    }
};

It cannot run.  need to skip the space and punctuation:

class Solution {
public:
    bool isPalindrome(string s) {

        int i = 0;
        int j = s.length() - 1;
        
        while (i < j) {
            // 跳过非字母数字字符
            if (!isalnum(s[i])) {
                i++;
                continue;
            }
            if (!isalnum(s[j])) {
                j--;
                continue;
            }
            
            // 比较字符，忽略大小写
            if (tolower(s[i]) != tolower(s[j])) {
                return false;
            }
            
            i++;
            j--;
        }
        
        return true;
        
    }
};

here, isalnum is a standard library. isalnum library to check if the input is numbers or letters. Think you receive a verifiction code, input the code.

!isalnum(s[j]) This expression checks if s[j] is not an alpha or numeric character. It returns true if s[j] is not an alpha or numeric, and false if it is, and is often used in string processing to skip non-alphanumeric characters.