◻️125. Valid Palindrome (easy)

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.

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.

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Solutions

Optimised (From Walkccc)

Runtime 4 ms Memory 8.94 MB

// Solution from https://walkccc.me/LeetCode/problems/125/
class Solution {
public:
    bool isPalindrome(string s) {
        int i=0, j=s.length()-1;
        while(i<j){
            while (i<j && !isalnum(s[i])){
                ++i;
            }
            while (i<j && !isalnum(s[j])){
                --j;
            }
            if(tolower(s[i]) != tolower(s[j])){
                return(false);
            }
            ++i; --j;
        }
        return(true);
    }
};

From Leetcode

Runtime 4 ms Memory 9.40 MB

// Solution from leetcode
class Solution {
public:
    bool isPalindrome(string s) {
        string str="";
        for(auto i: s){
            if(i>='A' && i<='Z'){
                str+=(i+32);
            }
            if(i>='a' && i<='z' || i>='0' && i<='9'){
                str+=i;
            }
        }
        int j=str.size()-1;
        for(int i=0; i<str.size()/2; i++){
            if(str[i] != str[j-i])
                return 0;
        }
        return 1;
    }
};