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def capturing_rainwater(heights):
waterTotal = 0
prevHeights = []
if heights:
maxHeight = heights[0]
else:
return 0
# Subtract preceding heights from current height (or max height if at a local maximum block to allow for local water levels) and remove negative and 0 values, the sum of this array results in the amount of water contained to the left by the current block
for i in heights.copy():
subArr = [i-a for a in prevHeights] if i < maxHeight else [maxHeight-a for a in prevHeights]
filterSub = [x for x in subArr if x > 0]
print(i, filterSub, subArr, prevHeights)
waterTotal += sum(filterSub)
# Increase heights of blocks that have already been counted as having some water above them up to this water level to avoid double counting
prevHeights = [i if x < i else x for x in prevHeights]
prevHeights.append(i)
# If at a local maximum block calculate the water for this local section (right of local maximum)
if i > maxHeight:
return waterTotal + capturing_rainwater(heights)
# Remove from unprocessed heights to allow for recersion
heights.pop(0)
return waterTotal
test_array = [4, 2, 1, 3, 0, 1, 2]
# test_array = [2,1,3,1,2]
# test_array = [3, 2, 1, 6, 5, 6, 2, 4]
print(capturing_rainwater(test_array))

def capturing_rainwater(heights):
# Write your code here
total = 0
for i in range(1,len(heights)-1):
left = []
#print(" ",i)
for it in reversed(range(i+1)):
if max(heights[i],heights[it])!= heights[i]:
left.append(max(heights[i],heights[it]))
right = []
for it in range(i,len(heights)):
if max(heights[i],heights[it])!=heights[i]:
right.append(max(heights[i],heights[it]))
if right == []:
right = [-1]
left = [-1]
if left == []:
left = [-1]
right = [-1]
right = max(right)
left = max(left)
#print(min(right,left))
#print(heights[i])
if right != -1 and left != -1:
total += min(right,left)-heights[i]
#print(total)
#print()
#print(left)
#print(right)
return total
test_array = [4, 2, 1, 3, 0, 1, 2]
print(capturing_rainwater(test_array))

Move one level up. Repeat this until you reach the upper value.

def capturing_rainwater(heights):
if len(heights) == 0: # When heights is []
return 0
else:
print(heights)
# Remove outer lower values:
if len(heights) > 2:
while heights[0] <= heights[1] and len(heights) > 2:
heights.pop(0)
while heights[-1] <= heights[-2] and len(heights) > 2:
heights.pop()
new_array = []
for x in heights:
new_array.append(x)
zeros = 0
for i in range(max(heights)):
# remove outer zeros:
while new_array[0] == 0:
new_array.pop(0)
while new_array[-1] == 0:
new_array.pop()
zeros += new_array.count(0) # count zeros between 'walls'
for j in range(len(new_array)): # voor elk item in regel:
# each value minus 1 (as if you go one level up)
if new_array[j] > 0:
new_array[j] = new_array[j] - 1
return zeros
test_array = [4, 2, 1, 3, 0, 1, 2]
print(capturing_rainwater(test_array))

Anyone know how to learn what inputs are being tested when you click Test Code? I got 4/5 tests passed, so I suspect there’s some edge case I didn’t handle, but I’m not sure what it is. I did discover that one of the five tests has an empty list as an input.

def capturing_rainwater(heights):
num_wells = len(heights) # the number of wells/bars
if num_wells <= 2: # ignore the edge cases
return 0
def big_left(i): # find the tallest bar to the left of index i
return max([heights[j]-heights[i] for j in range(0,i+1)])
def big_right(i): # find the tallest bar to the right of index i
return max([heights[j]-heights[i] for j in range(i,num_wells)])
count = 0
for i in range(1, num_wells - 1):
x = big_left(i)
y = big_right(i)
z = min(x,y) # the amount of water is the min of x and y
count += z
return count
test_array = [4, 2, 1, 3, 0, 1, 2]
print(capturing_rainwater(test_array))

I thought this was solution was easy to follow. The solution from @steffan153 was awesome, but hurt my brain. Logically, the number of water units above a given bar is the minimum of the tallest bars to the left and the right. Once you realize that, a solution is simple. Steffan showed how to simplify the code to its bare bones.

def capturing_rainwater(heights):
if len(heights) <= 2: return 0
return sum([min(max([heights[i]-heights[n] for i in range(0,n+1)]), max([heights[i]-heights[n] for i in range(n,len(heights))])) for n in range(1,len(heights)-1)])

I was toying with this problem again and did it in 2 lines. Not very readable, but kind of silly.

Edited to add: it only needs one line. The If isn’t necessary…

I did mine by finding an interval that could hold water (by finding the index of a maximum or the index of a relative maximum with a value above a specified number) and then finding the volume of water contained in such an interval. Repeat until reach the end of the list/array.

My code does not seem efficient … its longer than the other answers given.
I tried to avoid creating new lists or traversing the list unnecessarily.

My solution

def capturing_rainwater(heights):
length = len(heights)
# function to find next index
# of maximum of remaining entries in list or
# next relative maximum (peak) above a given hieght (level)
def get_next_max_index(starting_index = 0, level = None):
max_so_far = 0
index_of_max_so_far = starting_index
for i in range(starting_index, length):
h = heights[i]
if (h >= max_so_far):
index_of_max_so_far = i
max_so_far = h
if (level is not None):
if (h > level):
previous_height = h
for j in range(i + 1, length):
current_height = heights[j]
if current_height >= previous_height:
# increasing so maybe haven't gotten to peak yet
index_of_max_so_far = j
else: # current_height < previous_height:
# decreasing so passed peak already
break
previous_height = current_height
return index_of_max_so_far
return index_of_max_so_far
def get_volume_between(a, b): # a,b are indices
if (a > b):
(a, b) = (b, a)
elif (a == b):
return 0
if (a < 0 or b < 0):
return 0
if (a >= length or b >= length):
return 0
level = min(heights[a], heights[b]) # water level
volume_so_far = 0
for i in range(a + 1, b):
h = heights[i]
if (h >= 0 and h < level):
depth = level - h
volume_so_far += depth
return volume_so_far
# will find the volume, one interval at a time
left_index = -1
right_index = 0
volume = 0
level = 0 # water level so far (if before index of max)
while right_index < length:
right_index = get_next_max_index(left_index + 1, level) # find right end of interval
if right_index >= 0 and right_index < length:
level = heights[right_index]
volume += get_volume_between(left_index, right_index)
left_index = right_index # to move to next interval
return volume

I am facing an issue where the Test says “Tests failed to run due to an error: “list index out of range”. Check your code and try again.”. Anyone can debug it or tell me where the issue is? Because I can’t see it and I never came across it. I tried changing the heights list so many times in hope I do come across it.

def capturing_rainwater(heights):
units = 0 # The units of water gathered
left_max_height = []
l_max_height = heights[0] # Initial max height from left
# Starting from left - right max height array
for i in range(0, len(heights)): # The idea is compare the prev max height
if heights[i] > l_max_height: # with the current height
l_max_height = heights[i]
left_max_height.append(l_max_height) # Append to list
right_max_height = [] # Max height arrays from left and from right
r_max_height = heights[len(heights)-1] # Initial max height from right
# Starting from left - left max height array
for j in range(len(heights)-1, -1, -1):
if heights[j] > r_max_height:
r_max_height = heights[j]
right_max_height.append(r_max_height) # Create a max height from right
right_max_height.reverse() # Reverse list starting from right
# The minimum between the two maximum heights decides what
# can be gathered by the columns
for k in range(0, len(heights)-1):
units += min(left_max_height[k], right_max_height[k]) - heights[k]
return units
test_array = [0, 10, 5, 30, 10, 40]
print(capturing_rainwater(test_array))