Leetcode #1423: Maximum Points You Can Obtain from Cards (https://interviewrecipes.com/leetcode-1423)

Leetcode_1423.cpp

class Solution {
public:
    int maxScore(vector<int>& cardPoints, int k) {
        int n = cardPoints.size();
        vector<int> cumulativeSumFront(n+1, 0); // For easier indexing, add an extra element.
        vector<int> cumulativeSumBack(n+1, 0);  // For easier indexing, add an extra element.
        
        for (int i=0; i<n; i++) {
            cumulativeSumFront[i+1] = cumulativeSumFront[i] + cardPoints[i];
        }
        for (int i=n-1; i>=0; i--) {
            cumulativeSumBack[i] = cumulativeSumBack[i+1] + cardPoints[i];
        }
        
        // Reversing is optional. I reversed it so that it would be easy
        // to access sum of last (k-i) elements by just indexing at [k-i]
        // Otherwise, I would have had to index it at [n-k+i] which would
        // have made it difficult to read.
        reverse(cumulativeSumBack.begin(), cumulativeSumBack.end());
        
        int answer = 0;
        for(int i=0; i<=k; i++) {   
            answer = max(answer, 
                           cumulativeSumFront[i] // Sum of 'i' cards from the front.
                         + cumulativeSumBack[k-i]); // Sum of 'k-i' cards from the back.
        }
        return answer;
    }
};