go: add «3169. Count Days Without Meetings»

URL:	https://leetcode.com/problems/count-days-without-meetings/
Signed-off-by: Matej Focko <me@mfocko.xyz>

go/count-days-without-meetings.go

@@ -0,0 +1,27 @@
+package main
+
+import (
+	"cmp"
+	"slices"
+)
+
+func countDays(days int, meetings [][]int) int {
+	compareMeetings := func(a, b []int) int {
+		return cmp.Or(cmp.Compare(a[0], b[0]), cmp.Compare(a[1], b[1]))
+	}
+	slices.SortFunc(meetings, compareMeetings)
+
+	free, latestEnd := 0, 0
+	for _, meeting := range meetings {
+		start, end := meeting[0], meeting[1]
+
+		if start > latestEnd+1 {
+			free += start - latestEnd - 1
+		}
+
+		latestEnd = max(latestEnd, end)
+	}
+	free += days - latestEnd
+
+	return free
+}