C++ 递归与非递归方式实现中序遍历

用stack实现中序遍历

   TreeNode* KthNode(TreeNode* pRoot, int k)
    {
        if(pRoot == nullptr || k <= 0)
        {
            return nullptr;
        }
        vector<TreeNode*> vec;
        stack<TreeNode*> sta;
        TreeNode* pNode = pRoot;
        while(pNode != nullptr || !sta.empty())
        {
            while(pNode != nullptr)
            {
                sta.push(pNode);
                pNode = pNode->left;
            }
            if(!sta.empty())
            {
                pNode = sta.top();
                sta.pop();
                vec.push_back(pNode);
                pNode = pNode->right;
            }
        }
        int length = vec.size();
        if(k <= length)
        {
           return vec[k - 1];
        }
        return nullptr;

非递归方式（stack）实现

TreeNode* KthNode(TreeNode* pRoot, int k)
    {
        if(pRoot == nullptr || k <= 0)
        {
            return nullptr;
        }
        vector<TreeNode*> vec;
        stack<TreeNode*> sta;
        TreeNode* pNode = pRoot;
        while(pNode != nullptr || !sta.empty())
        {
            while(pNode != nullptr)
            {
                sta.push(pNode);
                pNode = pNode->left;
            }
            if(!sta.empty())
            {
                pNode = sta.top();
                sta.pop();
                vec.push_back(pNode);
                pNode = pNode->right;
            }
        }
        int length = vec.size();
        if(k <= length)
        {
           return vec[k - 1];
        }
        return nullptr;
    }