树的中序遍历

递归版：

    void dfs(vector<int> &res, TreeNode* root){
        if(root==NULL)
            return;
        dfs(res,root->left);
        res.push_back(root->val);
        dfs(res,root->right);
    }
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        if(root == NULL)
            return vector<int>();
        dfs(res,root);
        return res;
    }

迭代版：

    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        stack<TreeNode*> sta;
        while(root!=NULL || !sta.empty()){
            while(root!=NULL){
                sta.push(root);
                root = root->left;
            }
            root = sta.top();
            sta.pop();
            res.push_back(root->val);
            root = root->right;
        }
        return res;
    }