JavaScript Solution

 let count = 0
    let str = s.split('')
    for (let i = 0, j = n - 1; i <= j; i++, j--) {
        if (str[i] !== str[j]) count++
    }
    if (count > k) return -1
    for (let i = 0, j = n - 1; i <= j; i++, j--) {
        if (i === j) {
            if (k > 0) str[i] = '9'; break;
        }
        let available = (k - (str[i] !== '9') - (str[j] !== '9'))
        let needed = (count - (str[i] !== str[j]))
        if (available >= needed) {
            count -= (str[i] !== str[j])
            k -= (str[i] !== '9') + (str[j] !== '9')
            str[i] = '9', str[j] = '9'
        } else {
            if (str[i] != str[j])
                k--, count--;
            str[i] = str[j] = Math.max(str[i], str[j]);
        }
        // console.log(available, needed)
    }
    return str.join('')

Here is my solution in java, javascript, python, C, C++, Csharp HackerRank Highest Value Palindrome Solution

 public static String highestValuePalindrome(String s, int n, int k) {
                char c [] = s.toCharArray();  
                int N=n/2; int j=-1;
                boolean [] B = new boolean [N];
                for(int i=0;i<N; i++){
                        B[i]=c[i]==c[n-i-1];
                        if(!B[i]&&--k<0)return "-1";
                        c[i]=(char)(Math.max(c[i],c[n-i-1]));
                        c[n-1-i]=c[i];
                }
                while(k>0&&++j<N){
                        if(c[j]!='9'&&((B[j]&&k>1)||(k>0&&!B[j]))){
                                c[j]='9'; c[n-j-1]='9';
                                k=k+(B[j]?-2:-1);
                        }
                }
                if(n%2==1&&k>0)c[N]='9';
                return new String(c);
        }
}

Here Is the solution of Highest Value Palindrome Click Here

All test cases passed

def highestValuePalindrome`(s, n, k) arr = s.split('') while (k > 0 && n > 1) firstIndex = 0 lastIndex = n - 1 while (firstIndex < lastIndex) if (arr[firstIndex] != arr[lastIndex]) if (arr[firstIndex] < arr[lastIndex]) arr[firstIndex] = arr[lastIndex] else arr[lastIndex] = arr[firstIndex] end k -= 1 end firstIndex += 1 lastIndex -= 1 end n -= 2 end if (k > 0) midIndex = n / 2 if (n.odd? && arr[midIndex] != '9' && k >= 1) arr[midIndex] = '9' k -= 1 end firstIndex = 0 lastIndex = n - 1 while (firstIndex < lastIndex && k >= 2) if (arr[firstIndex] != '9') arr[firstIndex] = '9' arr[lastIndex] =