体内思路：
这是一个贪心算法，每次从森林中选取权值最小的两个数，较大的做为左孩子，较小的做为右孩子，生成一个新的根，权值等于孩子权值的和，然后将新的结点放入队列中，依次执行，直到队列只有一个元素位置，这个元素就是哈夫曼树的根。

代码如下：

#include <stdio.h>
#include <vector>
#include <string>
#include <map>
#include <algorithm>
using namespace std;

struct HuffmanNode {
    HuffmanNode(char k, int w) : key(k), weight(w) ,
                                 left(nullptr),
                                 right(nullptr)
    {}
    HuffmanNode(int w) : key('\0'), weight(w) ,
                                 left(nullptr),
                                 right(nullptr)
    {}
    char key;
    int weight;
    HuffmanNode * left;
    HuffmanNode * right;
};

class ComHuffmanNode {
public:
    bool operator()(HuffmanNode * e1, HuffmanNode * e2) {
        return e1->weight > e2->weight;
    }
};

class HuffmanTree {
public:
    HuffmanTree() {
        DecodeTree = nullptr;
    }
    ~HuffmanTree() {
        ClearDecodeTree();
    }

    void Input(const map<char, int> & mapCh) {
        vector<HuffmanNode*> vecHufNode;
        for (auto itr=mapCh.begin(); itr!=mapCh.end(); ++itr) {
            vecHufNode.push_back(new HuffmanNode(itr->first, itr->second));
        }

        make_heap(vecHufNode.begin(), vecHufNode.end(), ComHuffmanNode());

        while (vecHufNode.size() > 1) {
            HuffmanNode * right = vecHufNode.front();
            pop_heap(vecHufNode.begin(), vecHufNode.end(), ComHuffmanNode());
            vecHufNode.pop_back();

            HuffmanNode * left = vecHufNode.front();
            pop_heap(vecHufNode.begin(), vecHufNode.end(), ComHuffmanNode());
            vecHufNode.pop_back();

            HuffmanNode * parent = new HuffmanNode(left->weight + right->weight);
            parent->left = left;
            parent->right = right;

            vecHufNode.push_back(parent);
            push_heap(vecHufNode.begin(), vecHufNode.end(), ComHuffmanNode());
        }

        if (!vecHufNode.empty()) {
            DecodeTree = vecHufNode.front();
        }

        veccode.resize(std::numeric_limits<char>().max());

        string code;

        BuildCode(DecodeTree, code);

    }

    void ClearDecodeTree(HuffmanNode * pNode) {
        if (pNode == nullptr) return;

        ClearDecodeTree(pNode->left);
        ClearDecodeTree(pNode->right);
        delete pNode;
    }

    void ClearDecodeTree() {
        ClearDecodeTree(DecodeTree);
        DecodeTree = nullptr;
    }

    void BuildCode(HuffmanNode * pNode, string & code) {
        if (pNode->left == NULL) {
            veccode[pNode->key] = code;
            return ;
        }

        code.push_back('0');
        BuildCode(pNode->left, code);
        code.pop_back();
        code.push_back('1');
        BuildCode(pNode->right, code);
        code.pop_back();
    }

    string Decode(const string & strB) {
        string strC;

        HuffmanNode * pNode = DecodeTree;
        for (int i=0; i<strB.size(); ++i) {
            if (strB[i] == '0') {
                pNode = pNode->left;
            } else {
                pNode = pNode->right;
            }

            if (pNode->left == NULL) {
                strC.push_back(pNode->key);
                pNode = DecodeTree;
            }
        }

        return strC;
    }

    string GetCode(const string & strA) {
        string strB;
        for (int i=0; i<strA.size(); ++i) {
            strB += veccode[strA[i]];
        }

        return strB;
    }


private:
    HuffmanNode * DecodeTree;
    vector<string> veccode;
};

int main(int argc, char ** argv) {

    map<char, int> mapCh = {
            {'a', 50},
            {'b', 20},
            {'c', 40},
            {'d', 10},
            {'e', 50}
    };

    HuffmanTree huffTree;

    huffTree.Input(mapCh);

    string strA = "aaaaabdaeaeadc";

    puts(strA.c_str());

    string strB = huffTree.GetCode(strA);

    puts(strB.c_str());

    printf("%d, %d\n", strA.size() * 8, strB.size());

    string strC = huffTree.Decode(strB);

    puts(strC.c_str());

    return 0;
}

输出如下：

aaaaabdaeaeadc
0101010101110111010001000111110
112, 31
aaaaabdaeaeadc

Process finished with exit code 0