trapping-rain-water.py

# Time:  O(n)
# Space: O(1)

class Solution(object):
    # @param A, a list of integers
    # @return an integer
    def trap(self, A):
        result = 0
        top = 0
        for i in xrange(len(A)):
            if A[top] < A[i]:
                top = i

        second_top = 0
        for i in xrange(top):
            if A[second_top] < A[i]:
                second_top = i
            result += A[second_top] - A[i]

        second_top = len(A) - 1
        for i in reversed(xrange(top, len(A))):
            if A[second_top] < A[i]:
                second_top = i
            result += A[second_top] - A[i]

        return result

# Time:  O(n)
# Space: O(n)
class Solution2(object):
    # @param A, a list of integers
    # @return an integer
    def trap(self, A):
        result = 0
        stack = []

        for i in xrange(len(A)):
            mid_height = 0
            while stack:
                [pos, height] = stack.pop()
                result += (min(height, A[i]) - mid_height) * (i - pos - 1)
                mid_height = height

                if A[i] < height:
                    stack.append([pos, height])
                    break
            stack.append([i, A[i]])

        return result