双指针往中间移动，若lmax>rmax则hi--，否则lo++

#
# 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)
        if n <= 3:
            return 0
        lo, hi = 0, n-1
        lmax = arr[lo]
        rmax = arr[hi]
        ans = 0
        while lo <= hi:            
            lmax = max(lmax, arr[lo])            
            rmax = max(rmax, arr[hi])
            if lmax > rmax:
                ans += rmax-arr[hi]
                hi -= 1
            else:
                ans += lmax-arr[lo]
                lo += 1
        return ans