【leetcode】1423. Maximum Points You Can Obtain from Cards

题目如下：

There are several cards arranged in a row, and each card has an associated number of points The points are given in the integer array cardPoints.

In one step, you can take one card from the beginning or from the end of the row. You have to take exactly k cards.

Your score is the sum of the points of the cards you have taken.

Given the integer array cardPoints and the integer k, return the maximum score you can obtain.

Example 1:

Input: cardPoints = [1,2,3,4,5,6,1], k = 3
Output: 12
Explanation: After the first step, your score will always be 1. However, choosing the rightmost card first will maximize your total score. The optimal strategy is to take the three cards on the right, giving a final score of 1 + 6 + 5 = 12.

Example 2:

Input: cardPoints = [2,2,2], k = 2
Output: 4
Explanation: Regardless of which two cards you take, your score will always be 4.

Example 3:

Input: cardPoints = [9,7,7,9,7,7,9], k = 7
Output: 55
Explanation: You have to take all the cards. Your score is the sum of points of all cards.

Example 4:

Input: cardPoints = [1,1000,1], k = 1
Output: 1
Explanation: You cannot take the card in the middle. Your best score is 1. 

Example 5:

Input: cardPoints = [1,79,80,1,1,1,200,1], k = 3
Output: 202

Constraints:

• 1 <= cardPoints.length <= 10^5
• 1 <= cardPoints[i] <= 10^4
• 1 <= k <= cardPoints.length

解题思路：题目看起来很唬人，可以从最左边取，也可以从右边取。忽略中间过程，最后的结果肯定是左边取了x个，右边取了k-x个，其中x>=0 && x <=k。所以，只需要找出一个x，使得和最大即可。

代码如下：

class Solution(object):
    def maxScore(self, cardPoints, k):
        """
        :type cardPoints: List[int]
        :type k: int
        :rtype: int
        """
        count_right = sum(cardPoints[len(cardPoints) - k:])
        count_left = 0
        res = count_right

        inx = len(cardPoints) - k

        for i in range(k):
            count_left += cardPoints[i]
            count_right -= cardPoints[inx]
            inx += 1
            res = max(res,count_right + count_left)

        return res