changeXY - Recursive - Java & C#

Given a string, compute recursively (no loops) a new string where all the lowercase 'x' chars have been changed to 'y' chars.

changeXY("codex") → "codey"
changeXY("xxhixx") → "yyhiyy"
changeXY("xhixhix") → "yhiyhiy"
Java :

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public String changeXY(String str) { if(str.length() < 1) return ""; char temp = str.charAt(str.length()-1); if(temp == 'x') temp = 'y'; return changeXY(str.substring(0,str.length()-1))+temp; }

Test Result:

C#:

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public static String changeXY(String str) { if (str.Length < 1) return ""; Char temp = str[str.Length - 1]; if (temp == 'x') temp = 'y'; return changeXY(str.Substring(0, str.Length - 1)) + temp; }

Test Result :