#include <iostream>
#include <vector>
using namespace std;

int findMaxSubquenceSum(vector<int> a) {
	int len = a.size();
	/*
	 * prevMaxSum[i-1]表示i前某一元素交換到i上是i前的元素可達到最大值
	 * 	是prevMaxSum[i-1] 不是prevMaxSum[i]
	 */
	int* prevMaxSum = new int[len];
	/*
	 * backMaxSum[i]表示從i到len-1的元素所能達到的最大值
	 */
	int* backMaxSum = new int[len];

	int curMax = a[0];
	prevMaxSum[0] = a[0];

	for (int i = 1; i < len; ++i) {
		//now記錄最大元素
		curMax = max(curMax, a[i]);
		prevMaxSum[i] = max(curMax, a[i] + prevMaxSum[i - 1]);
	}

	backMaxSum[len - 1] = a[len - 1];
	int result = a[len - 1];
	for (int i = len - 2; i >= 0; --i) {
		backMaxSum[i] = max(backMaxSum[i + 1], 0) + a[i];
		result = max(backMaxSum[i], result);
	}
	for (int i = 1; i < len; ++i) {
		/*
		 * 當交換元素下標為i時的最大值: backMaxSum[i] - a[i] + prevMaxSum[i - 1]
		 * 這裡是 prevMaxSum[i - 1]和backMaxSum[i] - a[i]注意下標
		 */
		result = max(backMaxSum[i] - a[i] + prevMaxSum[i - 1], result);
	}

	return result;
}

/*由前向後交換,所以必須有反序列*/
int findMaxSubquenceSumWithSwap(vector<int> a) {
	int result = findMaxSubquenceSum(a);

	for (int i = 0, j = a.size() - 1; i < j; ++i, --j) {
		swap(a[i], a[j]);
	}
	return max(result, findMaxSubquenceSum(a));
}