918. Maximum Sum Circular Subarray

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

Rust Solution

struct Solution;

impl Solution {
    fn max_subarray_sum_circular(a: Vec<i32>) -> i32 {
        let n = a.len();
        let sum = a.iter().sum::<i32>();
        let mut prev_min = 0;
        let mut prev_max = 0;
        let mut min = std::i32::MAX;
        let mut max = std::i32::MIN;
        for i in 0..n {
            prev_min = a[i].min(prev_min + a[i]);
            min = min.min(prev_min);
            prev_max = a[i].max(prev_max + a[i]);
            max = max.max(prev_max);
        }
        if max < 0 {
            max
        } else {
            max.max(sum - min)
        }
    }
}

#[test]
fn test() {
    let a = vec![1, -2, 3, -2];
    let res = 3;
    assert_eq!(Solution::max_subarray_sum_circular(a), res);
    let a = vec![5, -3, 5];
    let res = 10;
    assert_eq!(Solution::max_subarray_sum_circular(a), res);
    let a = vec![3, -1, 2, -1];
    let res = 4;
    assert_eq!(Solution::max_subarray_sum_circular(a), res);
    let a = vec![3, -2, 2, -3];
    let res = 3;
    assert_eq!(Solution::max_subarray_sum_circular(a), res);
    let a = vec![-2, -3, -1];
    let res = -1;
    assert_eq!(Solution::max_subarray_sum_circular(a), res);
}

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