Trapping Rain Water | Python Fiddle

#! /usr/bin/env python2.7

# A proposed solution to https://leetcode.com/problems/trapping-rain-water/description/

def last_index(arr):
    """
    For a given array, a map indexed by unique values in the array
    whose values are the last index of the given value.
    e.g. [0,3,1,2,3,1]
    would return {0:0, 3:4, 1:5, 2:4}
    """
    ret = {}
    for idx, val in enumerate(arr):
        ret[val] = idx
    return ret

def rain_water(arr):
    """
    Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining.
    """

trapped = 0
    actionableMax = 0
    lastmax = last_index(arr)

for idx, val in enumerate(arr[:-1]):  # We use [:-1] because the last slot will never trap water
        if val > actionableMax:
            # This may be a new actionableMax **if** it's not the last time we see this value
            if lastmax[val] > idx:  # Nope, not the last time!
                actionableMax = val
        elif val < actionableMax:
            diff = actionableMax - val
            trapped += diff

return trapped

print rain_water([0,1,0,2,1,0,1,3,2,1,2,1])

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