greedy.clj

(defn remove-sirens [sirens]
  (let [sorted (sort-by :end sirens)]
    (loop [remaining sorted
           last-end Integer/MIN_VALUE
           keep-count 0]
      (if (empty? remaining)
        (- (count sirens) keep-count)
        (let [{:keys [start end]} (first remaining)]
          (if (>= start last-end)
            (recur (rest remaining) end (inc keep-count))
            (recur (rest remaining) last-end keep-count)))))))

output

#'user/remove-sirens
user=> (remove-sirens [{:start 1 :end 5}
                {:start 3 :end 7}
                {:start 6 :end 9}
                {:start 8 :end 10}])
2
user=> (remove-sirens [{:start 0 :end 3}
                {:start 2 :end 4}
                {:start 5 :end 7}
                {:start 6 :end 8}
                {:start 8 :end 10}])
2