二叉树的遍历

/*struct TreeNode {    int val;    struct TreeNode *left;    struct TreeNode *right;    TreeNode(int x) :            val(x), left(NULL), right(NULL) {    }};*/class TreeToSequence {public:    vector<vector<int> > convert(TreeNode* root) {        vector<vector<int> > output;        vector<int> iteration;        first(root,iteration);        output.push_back(iteration);        iteration.clear();        middle(root,iteration);        output.push_back(iteration);        iteration.clear();        last(root,iteration);        output.push_back(iteration);        iteration.clear();        return output;    }    void first(TreeNode* root,vector<int> &iteration)    {        stack<TreeNode*> store;        TreeNode* current=NULL;        store.push(root);        while(!store.empty())        {            current=store.top();            iteration.push_back(current->val);            store.pop();            if(current->right!=NULL)store.push(current->right);             if(current->left!=NULL)store.push(current->left);          }    }    void middle(TreeNode* root,vector<int> &iteration)    {        stack<TreeNode*> store;        TreeNode* current=root;        while(!store.empty()||current!=NULL)        {            if(current!=NULL)            {                store.push(current);                current=current->left;            }            else            {                current=store.top();                iteration.push_back(current->val);                current=current->right;                store.pop();            }        }    }    void last(TreeNode* root,vector<int> &iteration)    {        stack<TreeNode*> store1,store2;        TreeNode* current=NULL;        store1.push(root);        while(!store1.empty())        {            current=store1.top();            store2.push(current);            store1.pop();            if(current->left!=NULL)store1.push(current->left);            if(current->right!=NULL)store1.push(current->right);        }        while(!store2.empty())        {            iteration.push_back(store2.top()->val);            store2.pop();        }    }};