区间地图实施

Question

我在其中一家IT公司发送了一份关于C ++职位的申请。他们给我发了一份测试任务。

任务是实现间隔图分配操作。我向他们发送了我的解决方案，但它没有通过第二个要求（正确的行为）。他们没有给出反馈，而是说我的代码没有通过他们所有的测试。现在我想知道我能做错什么。当然，在发送我的解决方案之前，我做了一些测试，并且我能想到的每一个测试都通过了。

现在我无法入睡而不知道我在哪里搞砸了。

这是我的代码：

void assign (const K & keyBegin, const K & keyEnd, const V & val )
{
    if (!(keyBegin < keyEnd))
        return;
    auto nextInterval = --m_map.upper_bound(keyEnd);
    auto inserted1 = m_map.end();
    auto inserted2 = m_map.end();
    if (nextInterval->second == val)
        ++nextInterval;
    else if (nextInterval->first < keyEnd)
    {
        const V & nextValue = nextInterval->second;
        ++nextInterval;
        inserted1 = nextInterval = m_map.emplace_hint(nextInterval, keyEnd, nextValue);
    }
    try
    {
        auto prevInterval = nextInterval;
        --prevInterval;
        if (keyBegin < prevInterval->first)
            prevInterval = --m_map.upper_bound(keyBegin);
        if (!(prevInterval->second == val))
        {
            if (prevInterval->first < keyBegin)
            {
                ++prevInterval;
                inserted2 = prevInterval = m_map.emplace_hint(prevInterval, keyBegin, val);
            }
            else
            {
                auto beforePrev = prevInterval;
                --beforePrev;
                if (beforePrev != m_map.end() && beforePrev->second == val)
                    prevInterval = beforePrev;
                else
                {
                    auto hint = m_map.erase(prevInterval);
                    inserted2 = prevInterval = m_map.emplace_hint(hint, keyBegin, val);
                }
            }
        }
        m_map.erase(++prevInterval, nextInterval);
    }
    catch (...)
    {
        if (inserted1 != m_map.end())
            m_map.erase(inserted1);
        if (inserted2 != m_map.end())
            m_map.erase(inserted2);
        throw;
    }
}

你能帮我找个错误吗？

Answer 1

你通过递减地图的开头来获得UB：

auto beforePrev = prevInterval;
--beforePrev;

Demo

您的以下测试也很奇怪：

if (beforePrev != m_map.end()

beforePrev在递减时不能是end()。

似乎你可以用

替换那个块

prevInterval->second = val;
if (prevInterval != m_map.begin() && !((--prevInterval)->second == val)){
    ++prevInterval;
} 

Demo

Answer 2

Vivek的上述两个解决方案不起作用。这是我能想到的最简单，最短的实现：

if (not (keyBegin < keyEnd) )
    return;

auto beginLowerBound = m_map.lower_bound(keyBegin);
auto endLowerBound = m_map.lower_bound(keyEnd);

std::optional<std::pair<K,V>> additionalElement;

if (endLowerBound == m_map.end() || keyEnd < endLowerBound->first)
    additionalElement = std::pair(keyEnd, operator[]( keyEnd ));
else if (beginLowerBound == m_map.end() || keyBegin < beginLowerBound->first)
    additionalElement = std::pair(keyEnd, operator[]( keyBegin ));

if (beginLowerBound != m_map.end())
    m_map.erase(beginLowerBound, endLowerBound);

m_map.insert(std::pair(keyBegin, val));

if (additionalElement)
    m_map.insert(additionalElement.value());

Answer 3

首先编写测试代码： 这是为了测试类型要求

class Value {

char a;

public:
        Value(char _a){ a = _a; }

        bool operator==(const Value& _val) const;
        friend ostream& operator<<(ostream& os, const Value& val); //  ONLY FOR TESTING, NOT RELATED TO SOLUTION

};

bool Value::operator==( const Value& _val ) const{
return ( a == _val.a ) ;
}

// operator<< is implemented ONLY FOR TESTING PURPOSE
ostream& operator<<(ostream& os, const Value& val)
{
    os << val.a;
    return os;
}

class Key {

int a;

public:
        Key(int _a){ a = _a; }
        Key(){ a = 0; }
        bool operator<(const Key& _key) const;
        friend ostream& operator<<(ostream& os, const Key& _key); //  ONLY FOR TESTING, NOT RELATED TO SOLUTION


};


bool Key::operator<( const Key& _key ) const{
return ( a < _key.a ) ;
}


// operator<< is implemented ONLY FOR TESTING PURPOSE
ostream& operator<<(ostream& os, const Key& _key)
{
    os <<  _key.a;
    return os;
}

现在实施

void assign( K const& keyBegin, K const& keyEnd, V const& val ) {

        K last = m_map.rbegin()->first;
        K first = m_map.begin()->first;


        if (!(keyBegin < keyEnd) ) return;
        if ( keyBegin < first ) return;
        if( last < keyEnd) return ;

         for (auto it = m_map.begin(); it!=m_map.end(); ++it)
         {
                if((*it).first < keyBegin) continue;
                else
                {
                        (*it).second=val;
                        it++;
                        auto tmp=it;
                        while((*it).first < keyEnd){
                                it++;
                        }
                        m_map.erase(tmp, it);

                break;
                }
         }
    }

另一种解决方案

void assign( K const& keyBegin, K const& keyEnd, V const& val ) {

        K last = m_map.rbegin()->first;
        K first = m_map.begin()->first;


        if (!(keyBegin < keyEnd) ) return;
        if ( keyBegin < first ) return;
        if( last < keyEnd) return ;

        auto itr1 = m_map.find(keyBegin);
        auto itr2 = m_map.find(keyEnd);

        (*itr1).second=val;
        itr1++;

        m_map.erase(itr1,itr2);

         cout << endl << endl;

    }

Answer 4

if (!(keyBegin < keyEnd))
        return;
    auto upper_bound_end = m_map.upper_bound(keyEnd);
    auto upper_bound_end2 = upper_bound_end;
    upper_bound_end2--;     
    auto uper_bound_begin = m_map.upper_bound(keyBegin);        
    auto uper_bound_begin2 = uper_bound_begin;
    uper_bound_begin2--;
    bool flag = false;
    bool flag2 = false;
    if (!((uper_bound_begin2)->second == val))
        flag = true;
    V tmp;
    if (!((upper_bound_end2)->second == val))
    {
        flag2 = true;
        tmp = (upper_bound_end2)->second;
    }   
    for (auto it = uper_bound_begin; it != upper_bound_end;)
    {
        it = m_map.erase(it);
    }
    if (flag == true)
        m_map[keyBegin] = val;
    if (flag2 == true)
        m_map[keyEnd] = tmp;

Answer 5

完整的工作代码，以下代码位于main（）中 （但雇主说模板类型使用不正确）

#define _ITERATOR_DEBUG_LEVEL 2
#define _SECURE_SCL 1
#define _HAS_ITERATOR_DEBUGGING 1

#include <iostream>
#include <iomanip>
#include <cassert>
#include <map>

template<typename K, typename V>
class interval_map {    
    V m_valBegin;
    std::map<K, V> m_map;
public:
    // constructor associates whole range of K with val
    interval_map(V const& val)
        : m_valBegin(val)
    {}

    void assign(std::map<K, V> const& testMap) {
        m_map = testMap;
    }

    // Assign value val to interval [keyBegin, keyEnd).
    // Overwrite previous values in this interval.
    // Conforming to the C++ Standard Library conventions, the interval
    // includes keyBegin, but excludes keyEnd.
    // If !( keyBegin < keyEnd ), this designates an empty interval,
    // and assign must do nothing.


    // look-up of the value associated with key
    V const& operator[](K const& key) const {
        auto it = m_map.upper_bound(key);
        if (it == m_map.begin()) {
            return m_valBegin;
        }
        else {
            return (--it)->second;
        }
    }

    void print() {

        std::cout << '\n' << m_valBegin << '\n';
        for (auto it = m_map.begin(); it != m_map.end(); ++it) {
            std::cout << it->first << ", " << it->second << '\n';
        }
    }

    void assign(K const& keyBegin, K const& keyEnd, V const& val) {
                
        if (!(keyBegin < keyEnd)) return;
        
        if (m_map.empty()) {

            if (m_valBegin == val)
                return;

            m_map.insert({ keyBegin, val });
            m_map.insert({ keyEnd, m_valBegin});
            return;
        }

        auto startIt = m_map.lower_bound(keyBegin);
        bool doErase = true;

        if (startIt == m_map.end())
            doErase = false;

        auto endIt = m_map.upper_bound(keyEnd);

        auto upperVal{ m_valBegin };

        if (endIt == m_map.begin())
            doErase = false;                    
        else
            upperVal = (--endIt)->second;
                
        if (endIt != m_map.end())
            endIt++;

        if(doErase)
            m_map.erase(startIt, endIt);
                
        m_map.insert({ keyBegin, val });
        m_map.insert({ keyEnd, upperVal });

        // ensure canonical - further down

        startIt = m_map.lower_bound(keyBegin);
        
        assert(startIt->second == val);         
        
        // go back to prev interval (it can have value = val)
        if (startIt != m_map.begin()) 
        {
            startIt--;

            // then our inserted value is the first equal to val
            if (startIt->second != val) 
                startIt++;
        }
            
        // skip first repeating value (first one should be left - others deleted)
        if(!(startIt == m_map.begin() && val == m_valBegin))
            startIt++;           
                            
        while(startIt != m_map.end())
        {           
            auto tmpKey = startIt->first;

            if ((startIt++)->second == val)
                m_map.erase(tmpKey);
            else 
                break;
        }
                
    }
};

int main() {
    interval_map<int, char> mymap{ 'a' };

    //mymap.assign({ {10,'c'},{20,'a'},{30,'c'},{40,'i'},{50,'c'},{60,'i'} });  
    //mymap.assign(65, 68, 'z');    mymap.print();

    while (1) 
    {       
        mymap.print();
    
        int start, end;
        char ch;

        std::cout << "\n\n Enter start, end, char: ";
        std::cin >> start >> end >> ch;
        
        mymap.assign(start, end, ch);
    }
    
}