*That* interview question, explored

water.clj

(ns water
  (require [clojure.string :as s]))

(defn right-reductions [f l]
  "just like (reductions) but form the right"
  (reverse (reductions f (reverse l))))

(defn transpose [m]
  "Transpose a list of lists"
  (apply mapv vector m))

(defn fill [land]
  "The main water filling function"
  (map -
    (map min (reductions max land)
             (right-reductions max land))
    land))

; Some rendering stuff

(defn line [land water maxi]
  "Draw a line of water"
  (concat
    (repeat land "#")
    (repeat water "~")
    (repeat (- maxi land water) " ")))

(defn render [land]
  "Give some land, calculate the fill and render it"
  (let [water (fill land)
        maxi  (apply max land)]
    (->> (map line land (fill land) (repeat maxi))
         (transpose)
         (map (partial apply str))
         (reverse)
         (s/join "\n"))))

water.py

def scan(f, l):
  o = [l[0]]
  for i in l[1:]:
    o.append(f(o[-1], i)
  return o

def water(land):
  return map(min, scan(max, l), reversed(maxl(reversed(l))))

def fill(land):
  return map(lambda a, b: a - b, water(land), land)