R solution to twitter waterflow problem

waterflow.R

# 1 line
function(x) sum(head(tail(pmin(cummax(x),rev(cummax(rev(x))))-x,-1),-1))

# or with fancy ascii visualisation!
#
# > water(x)
# $water
# [1] 10
#
# $vis
#       [,1]     
#  [1,] "XX"     
#  [2,] "XXXXX"  
#  [3,] "XOOOO"  
#  [4,] "XXOOO"  
#  [5,] "XXXOO"  
#  [6,] "XXXXO"  
#  [7,] "XXXXXXX"
#  [8,] "XXXXXXX"
#  [9,] "XXXXXX" 
water <- function(x) {
  pools <- head(tail(pmin(cummax(x),rev(cummax(rev(x))))-x,-1),-1)
  p.vis <- matrix(do.call(paste0,lapply(list(list("X",x),list("O",c(0,pools,0))),function(x)do.call(function(s,x)lapply(x,function(x)paste0(rep(s,x),collapse="")),x)))) 
  list(water=sum(pools), vis=p.vis)
}