How to merge two functions with same conditions?

Make an additional function e.g. determineTargetVector(const std::string &str) which returns the corresponding vector, where you want to insert/remove an element, so you have no redundant conditions. Also its nice to have only a single reponsibility for each function.

For example:

std::vector<int> *determineTargetVector(const std::string &str)
{
    if (str == "apples")
        return &apples;
    else if (str == "oranges")
        return &oranges;
    else if (str == "lemons")
        .
        .
        .
    else
        //something invalid, to check for in superior function
        return nullptr;
}

The easiest solution might be to use a std::map for those vectors:

std::map<std::string,std::vector<int>> fruitVecs;

The key values of the map would be "apples", "oranges", "bananas" etc.

Thus you can easily access the corresponding vector for any operation through the map.

A code that selects the vector to be used and then perform the action can be used :

class MyClass
{
public:
    void addFruit(const std::string &str, int count)
    {
        auto vec = selectVector(str);
        if(vec != nullptr)
            addToVec(*vec, count);
        else
            std::cout << "Unknown Fruit : " << str << '\n';
    }
    void removeFruit(const std::string &str)
    {
        auto vec = selectVector(str);
        if(vec != nullptr)
            removeFromVec(*vec);
        else
            std::cout << "Unknown Fruit : " << str << '\n';
    }
private:

    std::vector<int> *selectVector(const std::string &str)
    {
        if (str == "apples")
            return &apples;
        else if (str == "oranges")
            return &oranges;
        else if (str == "lemons")
            return &lemons;
        else if (str == "melons")
            return &melons;
        else if (str == "bananas")
            return &bananas;
        else
            return nullptr;
    }

    void addToVec(std::vector<int> &vec, int count)
    {
        vec.push_back(count);
    }
    void removeFromVec(std::vector<int> &vec)
    {
        vec.pop_back();
    }
    std::vector<int> apples;
    std::vector<int> oranges;
    std::vector<int> lemons;
    std::vector<int> melons;
    std::vector<int> bananas;
};