JS:

Keys:

• Watch this video: https://www.youtube.com/watch?v=zeprQpwdCPA

• We are first checking: left and right values and moving towards the middle in a while loop… if something does NOT match, we make the change and record that change in our Processed array (i.e. this will tell us what INDEX has been changed) AND when we make a change we decrement value of K - 1; because K tracks how many changes we can make, so we are working backwards

• ones we created palindrome and made and recorded the changes in the Processed array, we need to check if K > 0, because we can now make further changes

• so if K > 0 we reset taht values of our pointers i(left) and j(right)

• while i <= j AND k> 0 we run the new while loop

• if str[i] !== 9 we can make a change here

• we first check has ANY of the values either left or right been changed? If YES, we make 1 change, and set both str[i] and str[j] to 9 and K - 1; cause 1 change if NOT, we check if K value is bigger than > 1, if so, we make str[i] and str[j] to 9 and K - 2, because we have now made 2 changes.

• at the end of every while loop we i++; j = j - 1;

• on second while we have end condition that if i === j && k >=1 - i.e. we can still make 1 more change, then str[i] = 9; break; this is for test case 6, where there is middle value and string is made up for 5 characters; because at this point we are in DEAD MIDDLE, so we need to set this middle value to 9; we can do that by either str[i] = 9; OR str[j]=9 makes no difference, since both i and j are dead middle at this stage

• finally, just return: s.join(‘’) and it works

function highestValuePalindrome(s, n, k) {

if (n === 1){
    return k == 1 ? '9' : '-1'
}

let firstEl = s[0];

if(k===0 && [...s].every(el=> el===firstEl)){
    console.log('here')
    return s;
}



let i = 0;
let j = n-1;

s = Array.from(s, el=> Number(el));
console.log(s);

let processed = Array(n).fill(false);
console.log("processed: ", processed);


while(i <= j){

    if(k===0){
        return '-1'
    }

    if(s[i] > s[j]){
        s[j] = s[i];
        processed[j] = true;
        k--;
    }

    if(s[i] < s[j]){
        s[i] = s[j];
        processed[i] = true;
        k--;
    }

    i++;
    j--;
    // end of while loop 1
}

console.log(s);
console.log(processed);
console.log(k);


// now handle any remaining conditions

if(k > 0){

    i = 0;
    j = n-1;



    while(i <=j && k > 0){


        if(s[i] != 9){
            if(processed[i] === true || processed[j] === true){
                s[i]=9;
                s[j]=9;
                k--;
            } else {
                if(k > 1){
                    s[i]=9;
                    s[j]=9;
                    k = k - 2;
                }
            }
        }

        i++;
        j--;

        if(i===j && k >=1){
            s[i]=9;
            break;
        }



        // end of while loop 2
    }



}


console.log(s.join(''));
console.log(processed);
console.log(k);

return s.join('')



// ! function end

}

let s1 = '3943'; let n1 = 4; let k1 = 1; // 3993;

let s2 = '777'; let n2 = 3; let k2 = 0;

highestValuePalindrome(s2,n2,k2);