Because a String is immutable, a String concatenation of the form str1+=str2 takes time that is proportional to the length of the resulting string, even if str2 is very short. What is the running time of the code below, assuming that the array has N items?

public static String toString( Object [ ] arr ) { String result = " ["; for( String s : arr ) result += s + " "; result += "]"; return result; }