zip zap java cchallenge

Hi,
I am working on below challenge
http://codingbat.com/prob/p180759
I tried my code as below

public String zipZap(String str) {

String str2= str.replaceAll("z.p","zp");
  return str2;
  
 
 
 
}

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I am getting below result

Expected      Run            
zipZap("zipXzap") → "zpXzp"      "zpXzp"      OK         
zipZap("zopzop") → "zpzp"      "zpzp"      OK         
zipZap("zzzopzop") → "zzzpzp"      "zzzpzp"      OK         
zipZap("zibzap") → "zibzp"      "zibzp"      OK         
zipZap("zip") → "zp"      "zp"      OK         
zipZap("zi") → "zi"      "zi"      OK         
zipZap("z") → "z"      "z"      OK         
zipZap("") → ""      ""      OK         
zipZap("zzp") → "zp"      "zp"      OK         
zipZap("abcppp") → "abcppp"      "abcppp"      OK         
zipZap("azbcppp") → "azbcppp"      "azbcppp"      OK         
zipZap("azbcpzpp") → "azbcpzp"      "azbcpzp"      OK         
other tests
OK         

how to  improve my approach, results and design of this challenge. How do i make a graphical venn or flow chart or some other relevant diagram to design it before writing code to decide best strategy?
 Please advise