Leetcode 125. valid palindrome

Solution 1. Just parse string into character array. Then, use another loop to compare the character with head to middle and tail to middle.

var isPalindrome = function(s) {

    // parse string into valid character array

    let tmp = [];

    s.split("").forEach(c=>{

        c = c.toLowerCase();

        if (c>="a" && c<="z") {

            tmp.push(c);

        } else if (c>="0" && c<="9") {

            tmp.push(c);

        }

    });

    // compare characters from head to middle and tail to middle

    let len = tmp.length;

    let [i,j] = [0,len-1];

    while (i < j) {

        if (tmp[i]!==tmp[j]) {

            return false;

        }

        i++;

        j--;

    }

    return true;

};

Solution 2. Optimize it. Only one while loop with one head index and the other tail index. That's the way to stand out.

var isPalindrome = function(s) {

    function getChar(c) {

        let ret = "";

        c = c.toLowerCase();

        if ( 

            (c >= "0" && c <= "9") ||

            (c >= "a" && c <= "z")

        ) {

            return c;

        }

        return ret;

    }

    let len =s.length;

    let [headIdx, tailIdx] = [0, len-1];

    let headChar="";

    let tailChar="";

    while (headIdx < tailIdx) {

        //a character from head, return blank charater if not valid

        if (headChar.length===0) {

            headChar = getChar(s[headIdx]);

        }

        //a character from tail

        if (tailChar.length===0) {

            tailChar = getChar(s[tailIdx]);

        }

        //check if both head chacter and tail character valid

        //then check they are equal

        if (

            headChar.length===1 &&

            tailChar.length===1

        ) {

            if (headChar===tailChar) {

                headChar = "";

                tailChar = "";

                headIdx++;

                tailIdx--;

            } else {

                return false;

            }

        } else {

            // if not a valid character, move to next index

            if (headChar.length===0) {

                headIdx++;

            }

            if (tailChar.length===0) {

                tailIdx--;

            }

        }

    }

    return true;

};