Calculate the accumulated water element by element consider the amount of water on i 'th bar, the  amount of water accumulated  on this particular  bar will be equal to min(left[i], right[i]) - arr[i] .
for ( $ i = 0 ; $ i < $ n ; $ i ++ ) $ water += min ( $ left [ $ i ] , $ right [ $ i ] ) - $ arr [ $ i ] ; return $ water ; }