New未调用，但已分配内存

我写了一个简单的Trie实现。下面是源代码:

#include <string>
#include <map>
typedef unsigned int uint;
class Trie {
public:
    class Node {
    public:
            Node(const char & _value);
            ~Node();
            char get_value() const;
            void set_marker(const uint & _marker);
            uint get_marker() const;
            bool add_child(Node * _child);
            Node * get_child(const char & _value) const;
            void clear();
    private:
            char m_value;
            uint m_marker;
            std::map<char, Node *> m_children;
    };
    Trie();
    ~Trie();
    bool insert(const std::string & _str);
    bool find(const std::string & _str) const;
private:
    Node * m_root;
};
// - implementation (in a different file)
using namespace std;
Trie::Node::Node(const char & _value) :
            m_value(_value), m_marker(0), m_children() {
}
Trie::Node::~Node() {
    clear();
}
void Trie::Node::clear() {
    map<char, Node*>::const_iterator it;
    for (it = m_children.begin(); it != m_children.end(); ++it) {
            delete it->second;
    }
}
void Trie::Node::set_marker(const uint & _marker) {
    m_marker = _marker;
}
uint Trie::Node::get_marker() const {
    return m_marker;
}
char Trie::Node::get_value() const {
    return m_value;
}
Trie::Node * Trie::Node::get_child(const char & _value) const {
    map<char, Node*>::const_iterator it;
    bool found = false;
    for (it = m_children.begin(); it != m_children.end(); ++it) {
            if (it->first == _value) {
                    found = true;
                    break;
            }
    }
    if (found) {
            return it->second;
    }
    return NULL;
}
bool Trie::Node::add_child(Node * _child) {
    if (_child == NULL) {
            return false;
    }
    if (get_child(_child->get_value()) != NULL) {
            return false;
    }
    m_children.insert(pair<char, Node *>(_child->get_value(), _child));
    return true;
}
Trie::Trie() :
            m_root(new Node('')) {
}
Trie::~Trie() {
    delete m_root;
}
bool Trie::insert(const string & _str) {
    Node * current = m_root;
    bool inserted = false;
    for (uint i = 0; i < _str.size(); ++i) {
            Node * child = current->get_child(_str[i]);
            if (child == NULL) {
                    child = new Node(_str[i]);
                    current->add_child(child);
                    inserted = true;
            }
            current = child;
    }
    if (current->get_marker() != _str.size()) {
            current->set_marker(_str.size());
            inserted = true;
    }
    return inserted;
}
bool Trie::find(const std::string & _str) const {
    Node * current = m_root;
    bool found = false;
    for (uint i = 0; i < _str.size(); ++i) {
            Node * child = current->get_child(_str[i]);
            if (child == NULL) {
                    break;
            } else {
                    current = child;
            }
    }
    if (current->get_marker() == _str.size()) {
            found = true;
    }
    return found;
}

下面是我的测试程序:

#include <iostream>
#include <sstream>
#include "Trie.h"
int main() {
    Trie t;
    for (unsigned int i = 0; i < 10000; ++i) {
            t.insert("hello");
    }
    return 0;
}

我的问题是，即使'hello'在第二次尝试插入时已经插入，并且因此不再调用new，大量内存正在分配和取消分配。这个数量随着I增加max I的值而增加。例如，在上面的例子中，valgrind给出了这样的输出:

==10322== HEAP SUMMARY:
==10322==     in use at exit: 0 bytes in 0 blocks
==10322==   total heap usage: 10,011 allocs, 10,011 frees, 300,576 bytes allocated

我已经确认调用Node()构造函数的次数是恒定的。那么为什么以及如何分配和释放所有内存呢?