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`

918. Maximum Sum Circular Subarray
``````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);
}
``````