p4.clj

(ns euler.p4
  "A palindromic number reads the same both ways. The largest palindrome made
  from the product of two 2-digit numbers is 9009 = 91 99.

  Find the largest palindrome made from the product of two 3-digit numbers.")

(defn palindromic?
  [n]
  (loop [s (seq (str n))]
    (if (empty? s)
      true
      (if (= (first s) (last s))
        (recur (butlast (rest s)))
        false))))

(defn largest-palindromic-pair
  [n]
  (->>
    (for [x (range 11 n 11) y (range n 1 -1)
          :let [p (* x y)]
          :when (palindromic? p)]
      [p x y])
    (apply max-key first)
    rest))

(time (largest-palindromic-pair 999)) ; => [993 913]