It is similar to other buy/sell problems. just do DP and define an array of states to track the current maximum profits at different stages. For example, in the below code

- states[][0]: one buy
- states[][1]: one buy, one sell
- states[][2]: two buys, one sell
- states[][3]: two buy, two sells

The states transistions occurs when buy/sell operations are executed. For example, state[][0] can move to state[][1] via one sell operation.

```
class Solution {
public:
int maxProfit(vector<int>& prices) {
int states[2][4] = {INT_MIN, 0, INT_MIN, 0}; // 0: 1 buy, 1: one buy/sell, 2: 2 buys/1 sell, 3, 2 buys/sells
int len = prices.size(), i, cur = 0, next =1;
for(i=0; i<len; ++i)
{
states[next][0] = max(states[cur][0], -prices[i]);
states[next][1] = max(states[cur][1], states[cur][0]+prices[i]);
states[next][2] = max(states[cur][2], states[cur][1]-prices[i]);
states[next][3] = max(states[cur][3], states[cur][2]+prices[i]);
swap(next, cur);
}
return max(states[cur][1], states[cur][3]);
}
};
```