Question #3
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// Return a string that is equal to the given string s with all occurrences of pat
//replaced by new.

public static String substitute(String s, String pat, String newS)
{
    String t = ””;  // Result so far.
    int p;	        // Location of leftmost occurrence of pat in s, or –1 if none.

    p = s.indexOf (pat);


    while (p!=-1)
    {
         t = t+s.substring(0,p) + newS;


         s = s.substring( p + pat.length(), s.length());


         p = s.indexOf(pat);
    }

   return t+s ;
}