[leetcode]Best Time to Buy and Sell Stock III

Say you have an array for which the ith element is the price of a given stock on day i.

Design an algorithm to find the maximum profit. You may complete at most two transactions.

Note:
You may not engage in multiple transactions at the same time (ie, you must sell the stock before you buy again).

class Solution {
public:
int maxProfit(vector<int> &prices) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
if(prices.empty()) return 0;
int N = prices.size();

vector<int> forward(N, 0);
vector<int> backward(N, 0);

int curmin = prices[0];
int result = 0;

for(int i = 1; i < prices.size(); i++){
curmin = min(curmin, prices[i]);
result = max(result, prices[i] - curmin);

forward[i] = result;
}

int curmax = prices.back();
result = 0;
for(int i = N-2; i >= 0; i--){
curmax = max(curmax, prices[i]);
result = max(result, curmax - prices[i]);

backward[i] = result;
}

int ret = 0;
for(int i = 0; i < N-1; i++){
ret = max(ret, forward[i] + backward[i+1]);
}

ret = max(ret, forward.back());

return ret;
}
};