add the dna algorithm (#6323)

* adding the dna algorithm

* following bot recommendations

following bot recommendations for the indentation

* following bot recommendations

following bot recommendations regarding indentation [ again ]

* following bot recommendations

following bot recommendations regarding indentation [ again. ]

* following bot recommendations

following bot recommendations.

strings/dna.py

@@ -0,0 +1,26 @@
+import re
+
+
+def dna(dna: str) -> str:
+
+    """
+    https://en.wikipedia.org/wiki/DNA
+    Returns the second side of a DNA strand
+
+    >>> dna("GCTA")
+    'CGAT'
+    >>> dna("ATGC")
+    'TACG'
+    >>> dna("CTGA")
+    'GACT'
+    >>> dna("GFGG")
+    'Invalid Strand'
+    """
+
+    r = len(re.findall("[ATCG]", dna)) != len(dna)
+    val = dna.translate(dna.maketrans("ATCG", "TAGC"))
+    return "Invalid Strand" if r else val
+
+
+if __name__ == "__main__":
+    __import__("doctest").testmod()