Notes

125. Valid Palindrome

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.


Valid Palindrome - compare against reverse
const string = "A man, a plan, a canal: Panama"

const isValidPalindrome = function(s) {
s = s.replace(/[^A-Za-z0-9]/g, '').toLowerCase();
let rev = "";

// generate a reverse string using a reverse for loop.
for(let i = s.length - 1; i >= 0; i--) {
rev += s[i];
}

return rev === s;
};

console.log(isValidPalindrome(string));

Valid Palindrome - 2 pointers from outside
const string = "A man, a plan, a canal: Panama"

const isValidPalindrome = function(s) {
s = s.replace(/[^A-Za-z0-9]/g, '').toLowerCase();

// initialize left/right pointers at start and end of string respectively
let left = 0; right = s.length - 1;

// loop through string characters while comparing them, then move the pointers closer to the center
while(left < right) {
if(s[left] !== s[right]) {
return false
}

left++;
right--;
}

return true;
};

console.log(isValidPalindrome(string));

Valid Palindrome - 2 pointers from center

rom const string = "A man, a plan, a canal: Panama"

const isValidPalindrome = function(s) {
s = s.replace(/[^A-Za-z0-9]/g, '').toLowerCase();

// initialize left/right pointers to point at the middle index of the string. Remember, indexes start at 0 meaning that we have to floor() the value from dividing length by 2 in order to get the index of the center.
let left = Math.floor(s.length / 2), right = left;

// if the string is even, move left pointer back by 1 so left/right are pointing at the 2 middle values respectively.
if(s.length % 2 === 0) {
left--;
}

// loop through the string while expanding pointers outwards comparing the characters.
while(left >= 0 && right < s.length) {
if(s[left] !== s[right]) {
return false
}

left--;
right++;
}

return true;
};

console.log(isValidPalindrome(string));


Almost palindrome
Given a string s, return true if the s can be palindrome after deleting at most one character from it.

var validPalindrome = function(s) {
let start = 0;
let end = s.length - 1;
while (start < end) {
if (s[start] !== s[end]) {
return validSubPalindrome(s, start + 1, end) || validSubPalindrome(s, start, end - 1);
}
start++;
end--;
}
return true;
};

var validSubPalindrome = function(s, start, end) {
while (start < end) {
if (s[start] !== s[end]) {
return false;
}
start++;
end--;
}
return true;
};