LC 918. Maximum Sum Circular Subarray

Posted 2021-02-14 ethanhong

Given a circular array C of integers represented by A, find the maximum possible sum of a non-empty subarray of C.

Here, a circular array means the end of the array connects to the beginning of the array.  (Formally, C[i] = A[i] when 0 <= i < A.length, and C[i+A.length] = C[i] when i >= 0.)

Also, a subarray may only include each element of the fixed buffer A at most once.  (Formally, for a subarray C[i], C[i+1], ..., C[j], there does not exist i <= k1, k2 <= j with k1 % A.length = k2 % A.length.)

 

Example 1:

Input: [1,-2,3,-2]
Output: 3
Explanation: Subarray [3] has maximum sum 3

Example 2:

Input: [5,-3,5]
Output: 10
Explanation: Subarray [5,5] has maximum sum 5 + 5 = 10

Example 3:

Input: [3,-1,2,-1]
Output: 4
Explanation: Subarray [2,-1,3] has maximum sum 2 + (-1) + 3 = 4

Example 4:

Input: [3,-2,2,-3]
Output: 3
Explanation: Subarray [3] and [3,-2,2] both have maximum sum 3

Example 5:

Input: [-2,-3,-1]
Output: -1
Explanation: Subarray [-1] has maximum sum -1

 

Note:

1. -30000 <= A[i] <= 30000
2. 1 <= A.length <= 30000

 

 

 

Runtime: 76 ms, faster than 100.00% of C++ online submissions for Maximum Sum Circular Subarray.

Memory Usage: 13.2 MB, less than 0.77% of C++ online submissions for Maximum Sum Circular Subarray.

 

class Solution {
public:
  int maxSubarraySumCircular(vector<int>& A) {
    int total = 0;
    int curmax = 0, maxsum = INT32_MIN;
    int curmin = 0, minsum = INT32_MAX;
    for(int a : A) {
      curmax = max(curmax + a, a);
      maxsum = max(maxsum, curmax);
      curmin = min(curmin + a, a);
      minsum = min(minsum, curmin);
      total += a;
    }
    return maxsum > 0 ? max(maxsum, total - minsum) : maxsum;
  }
};