【leetcode42】接雨水

双指针法，左侧更低则考虑左侧，右侧更低则考虑右侧

 def trap(height):
    left = 0
    right = len(height)-1
    left_max = 0
    right_max = 0
    current_water = 0
    while left<=right:
        if height[left]<=height[right]:
            if height[left]>left_max:
                left_max = height[left]
            else:
                current_water+=left_max-height[left]
            left+=1
        else:
            if height[right]>right_max:
                right_max = height[right]
            else:
                current_water+=right_max-height[right]
            right-=1
    return current_water