Max Increase to Keep City Skyline

LeetCode 807.Max Increase to Keep City Skyline

Medium

In a 2 dimensional array grid, each value grid[i][j] represents the height of a building located there. We are allowed to increase the height of any number of buildings, by any amount (the amounts can be different for different buildings). Height 0 is considered to be a building as well.

At the end, the "skyline" when viewed from all four directions of the grid, i.e. top, bottom, left, and right, must be the same as the skyline of the original grid. A city's skyline is the outer contour of the rectangles formed by all the buildings when viewed from a distance. See the following example.

What is the maximum total sum that the height of the buildings can be increased?

Example:
Input: grid = [[3,0,8,4],[2,4,5,7],[9,2,6,3],[0,3,1,0]]
Output: 35
Explanation: 
The grid is:
[ [3, 0, 8, 4], 
  [2, 4, 5, 7],
  [9, 2, 6, 3],
  [0, 3, 1, 0] ]

The skyline viewed from top or bottom is: [9, 4, 8, 7]
The skyline viewed from left or right is: [8, 7, 9, 3]

The grid after increasing the height of buildings without affecting skylines is:

gridNew = [ [8, 4, 8, 7],
            [7, 4, 7, 7],
            [9, 4, 8, 7],
            [3, 3, 3, 3] ]

Notes:

· 1 < grid.length = grid[0].length <= 50.

· All heights grid[i][j] are in the range [0, 100].

· All buildings in grid[i][j] occupy the entire grid cell: that is, they are a 1 x 1 x grid[i][j] rectangular prism.

Python版代码（未优化）：

class Solution:
    def maxIncreaseKeepingSkyline(self, grid: List[List[int]]) -> int:
        sums = 0
        for i in range(len(grid)):
           
            for j in range(len(grid[i])):
                temp2 = max([grid[m][j] for m in range(len(grid[i]))])  # up\down
                temp3 = max(grid[i])  #  right\left
                mins = temp3 if temp2>temp3 else temp2
                if grid[i][j]<mins:
                    sums +=(mins - grid[i][j])
        return sums

运行结果

Python版代码（优化后）：

class Solution:
    def maxIncreaseKeepingSkyline(self, grid: List[List[int]]) -> int:
        sums = 0
        temp2 = [max(i) for i in grid]
        temp3 = [max(i) for i in zip(*grid)]
        
        for i in range(len(grid)):
           
            for j in range(len(grid[i])):
                
                mins = temp3[j] if temp2[i]>temp3[j] else temp2[i]
                if grid[i][j]<mins:
                    sums +=(mins - grid[i][j])
        return sums

运行截图

可以看出优化后，运行速度提高了3倍左右，占用内存差不多。