lmax是arr[0...lo]中最高柱子的高度，rmax是arr[hi...]中最高柱子的高度

#
# max water
# @param arr int整型一维数组 the array
# @return long长整型
#
class Solution:
    def maxWater(self , arr ):
        # write code here
        n = len(arr)        
        lo, hi = 0, n-1
        lmax, rmax = arr[0], arr[n-1]
        res = 0
        while lo <= hi:
            lmax = max(lmax, arr[lo])
            rmax = max(rmax, arr[hi])

            if lmax > rmax:
                res += max(0, (rmax-arr[hi]))
                hi -= 1
            else:
                res += max(0, (lmax-arr[lo]))
                lo += 1
        return res