【基础操作】先序后序中序三种方式非递归实现遍历二叉树

要吐了要吐了，深更半夜太晚了就先不写思路了，直接上代码：

vector<int> preorderTraversal(TreeNode *root, vector<int>path)
{
    stack<TreeNode*>TreeNodeStack;
    if(root==NULL)
        return path;
    TreeNode* pNode=root;
    TreeNodeStack.push(pNode);
    while(!TreeNodeStack.empty()){
        pNode=TreeNodeStack.top();
        path.push_back(pNode->val);
        TreeNodeStack.pop();
        if(pNode->right){
            TreeNodeStack.push(pNode->right);
        }
        if(pNode->left){
            TreeNodeStack.push(pNode->left);
        }
    }
};
vector<int> inorder(TreeNode* root, vector<int> path){
    stack<TreeNode*>TreeNodeStack;
    TreeNode* pNode=root;
    while(pNode||!TreeNodeStack.empty()){
        while(pNode){
            TreeNodeStack.push(pNode);
            pNode=pNode->left;
        }
        if(!TreeNodeStack.empty()){
            pNode=TreeNodeStack.top();
            TreeNodeStack.pop();
            path.push_back(pNode->val);
            pNode=p->right;
        }
    }
    return path;
}
vector<int> postorderTravelsal(vector<int> path, TreeNode* root){
        if(root==NULL){
            return path;
        }
        TreeNode* pPrev=NULL;
        TreeNode* pNode=root;
        stack<TreeNode*>TreeNodeStack;
        while(pNode||!TreeNodeStack.empty()){
            while(pNode){
                TreeNodeStack.push(pNode);
                pNode=pNode->left;
            }
            pNode=TreeNodeStack.top();
            if(pNode->right==NULL||pNode->right==pPrev){
                path.push_back(pNode->val);
                pPrev=pNode;
                TreeNodeStack.pop();
                pNode=NULL;
            }
            else{
                pNode=pNode->right;
            }
        }
        return path;
    }