How to code Z-Function of string?

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I cannot make a code that is calculating Z-Function of string. 
Main trouble is that string is immutable char[].
1. How can I make string ONLY char[] (Not immutable)
2. How can I work with some of chars in the stirng, is, for 
example:
    string s="abc";
    writeln(s[1]); // Should write 'b' or not???
Is it a legit code or it doesn't work ?