LeetCode - 1191. K-Concatenation Maximum Sum

LeetCode - 1191. K-Concatenation Maximum Sum

int maxSubArray(const vector<int>& nums) {
	int max_sum = INT_MIN, cur_sum = 0;
	for (const int& i : nums) {
		cur_sum = max(i, cur_sum + i);
		max_sum = max(max_sum, cur_sum);
	}
	return max_sum;
}

int kConcatenationMaxSum(vector<int> arr, int k) {
	const int MOD = 1e9 + 7, len = arr.size();
	int sum = accumulate(arr.begin(), arr.end(), 0);
	// 計算左半部分數組最大和
	int cur = 0, l_max = 0, r_max = 0;
	for(int i = 0; i < len; ++i){
		cur += arr[i];
		l_max = max(l_max, cur);
	}
	l_max = max(l_max, 0) % MOD;
	// 計算右半部分數組最大和
	cur = 0;
	for (int i = len - 1; i >= 0; --i) {
		cur += arr[i];
		r_max = max(r_max, cur);
	}
	r_max = max(r_max, 0) % MOD;
	// 計算數組中最大連續子數組
	const int max_sub = maxSubArray(arr) % MOD;
	if (k == 1)	// 1 <= k <= 10^5
		return max({max_sub, sum % MOD, l_max, r_max});
    sum = sum > 0 ? (((long long)sum * (k - 2)) % MOD) : 0;
	const int max_sum = (sum + l_max + r_max) % MOD;
	return max(max_sub, max_sum);
}