go: add «409. Longest Palindrome»

Signed-off-by: Matej Focko <me@mfocko.xyz>

go/longest-palindrome.go

@@ -0,0 +1,26 @@
+package longest_palindrome
+
+func longestPalindrome(s string) int {
+	getFreqs := func() map[rune]int {
+		freqs := make(map[rune]int)
+		for _, c := range s {
+			freqs[c]++
+		}
+		return freqs
+	}
+	freqs := getFreqs()
+
+	length := 0
+
+	usedOdd := false
+	for _, count := range freqs {
+		length += 2 * (count / 2)
+
+		if !usedOdd && count%2 == 1 {
+			length += 1
+			usedOdd = true
+		}
+	}
+
+	return length
+}