我的 C++ 代码中有一个奇怪的错误

Question

Our professor want us to write some code to do some calculate about financial instrument, buy low and sell high.and the prices change as time. for example, the input is {1,2,4} so we are able to buy 1 and sell as 2, and buy 2 sell 4. so the output should be 1+2=3. And if the input is {4,2,1} we are not able to buy low and sell high. so the out put should be 0. and here's my code.

#include<iostream>
#include<vector>

using namespace std;

int getMaxProfit(vector<int> &prices)
 {
int p;
int n=0;
vector<int> profits;

if(prices.size()==1)
{
    return 0;
}

for(int i=0; i<prices.size();i++)
{
    if (prices[i]<prices[i+1])
    {
        p=prices[i+1]-prices[i];
        profits.push_back(p);
    }

}

for(int i=0; i<profits.size();i++)
{
    n=n+profits[i];

}


return n;
}

int main()
{
vector<int> prices = {1,2,4};

cout<<getMaxProfit(prices);

return 0;
}

it works pretty well when there's only 4 or less value in my input. However if there's more than 4 values, like {1,1,2,3,4}. I will get the wrong answer. I cant fig out what's wrong about it.

Answer 1

Ok, I got it. Changed a little about my code and it worked.

#include<iostream>
#include<vector>

using namespace std;

int getMaxProfit(vector<int> &prices)
{
int p;
int n=0;
vector<int> profits;

if(prices.size()==1)
{
    return 0;
}
p = prices[0];
for(int i=1; i<prices.size();i++)
{
    if (p<prices[i])
    {
        n=prices[i]-p;
        profits.push_back(n);
        p=prices[i];
    }
    else
    {
        p =prices[i];
    }
}
n=0;

for(int i=0; i<profits.size();i++)
{
    n=n+profits[i];

}


return n;
}

int main()
{
vector<int> prices = {3,1,5,2,4};

cout<<getMaxProfit(prices);

return 0;
}