Time Complexity : O(n) Space Complexity: O(1)

Java 8

// a-z 97-122 // A-Z 65- 90

    if(k==0){ return s;}

    k=k%26;

    int A = 65;
    int Z = 90;
    int a = 97;
    int z = 122;

    for(int i=0; i<s.length(); i++){

        int currentChar = s.charAt(i);
        int newChar = currentChar + k;
        char intToCharConversion;
        if(currentChar >= A && currentChar <= Z){
            if(newChar > Z){
                int updatedChar = (A - 1)+ (newChar - Z);
                intToCharConversion = (char)updatedChar;
                s =  s.substring(0, i) + intToCharConversion + s.substring(i + 1);
            }else{
                intToCharConversion = (char)newChar;
                s =  s.substring(0, i) + intToCharConversion + s.substring(i + 1);
            }
        }else if(currentChar >= a && currentChar <= z){
            if(newChar > z){
                int updatedChar = (a - 1)+ (newChar - z);
                intToCharConversion = (char)updatedChar;
                s =  s.substring(0, i) + intToCharConversion + s.substring(i + 1);
            }else{
                intToCharConversion = (char)newChar;
                s =  s.substring(0, i) + intToCharConversion + s.substring(i + 1);
            }
        }
    }    

    return s;