二叉树递归/非递归遍历

/*递归遍历*/

//前序遍历
void preorder(TreeNode *root, vector<int> &path){
    if(root != NULL)
    {
        path.push_back(root->val);
        preorder(root->left, path);
        preorder(root->right, path);
    }
}
//中序遍历
void inorder(TreeNode *root, vector<int> &path){
    if(root != NULL)
    {
        inorder(root->left, path);
        path.push_back(root->val);
        inorder(root->right, path);
    }
}

//后续遍历
void postorder(TreeNode *root, vector<int> &path){
    if(root != NULL)
    {
        postorder(root->left, path);
        postorder(root->right, path);
        path.push_back(root->val);
    }
}

/*非递归遍历*/

//非递归前序遍历
void preorderTraversal(TreeNode *root, vector<int> &path){
    stack<TreeNode *> s;
    TreeNode *p = root;
    while(p != NULL || !s.empty())
    {
        while(p != NULL)
        {
            path.push_back(p->val);
            s.push(p);
            p = p->left;
        }
        if(!s.empty())
        {
            p = s.top();
            s.pop();
            p = p->right;
        }
    }
}
//非递归中序遍历
void inorderTraversal(TreeNode *root, vector<int> &path){
    stack<TreeNode *> s;
    TreeNode *p = root;
    while(p != NULL || !s.empty())
    {
        while(p != NULL)
        {
            s.push(p);
            p = p->left;
        }
        if(!s.empty())
        {
            p = s.top();
            path.push_back(p->val);
            s.pop();
            p = p->right;
        }
    }
}
//非递归后序遍历-迭代
void postorderTraversal(TreeNode *root, vector<int> &path)
{
    stack<TreeNode *> s;
    TreeNode *p = root;
    TreeNode *pre = NULL;//保存上一次访问到的结点
    while(p != NULL || !s.empty())
    {
        while(p != NULL) //沿左子树一直往下搜索，直至出现没有左子树的结点
        {
            s.push(p);
            p = p->left;
        }
        if(!s.empty())
        {
            p = s.top();
            if(p->right == NULL || p->right == pre)   //表明p的右子树都访问过了,接下来就是访问这个结点了
            {
                path.push_back(p->val);
                pre = p;
                s.pop(); p = NULL;//目的就是不让s再次压入p的左子树们
            }
            else
            {
                p = p->right;
            }
        }
    }
}