/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
 //二叉树的前序遍历 moris写法。
class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> res;
        if (!root) return res;
        while (root) {
            if (root->left) {
                auto p = root->left;
                while (p->right && p->right != root) p = p->right;
                if (!p->right) {
                    res.push_back(root->val);
                    p->right = root;
                    root = root->left;
                } else {
                    p->right = NULL;
                    root = root->right;
                }
            } else {
                res.push_back(root->val);
                root = root->right;
            }
        }
        return res;
    }
};

// 二叉树的中序遍历 moris写法
class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        vector<int> res;
        auto cur = root;
        while (cur) {
            if (cur->left) {
                auto l = cur->left;
                while (l->right && l->right != cur) l = l->right;
                if (!l->right) {
                    l->right = cur;
                    cur = cur->left;
                } else {
                    res.push_back(cur->val);
                    l->right = NULL;
                    cur = cur->right;
                }
            } else {
                res.push_back(cur->val);
                cur = cur->right;
            }
        }
        return res;
    }
};

// 二叉树的后续遍历 moris写法。

class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
        vector<int> res;
        if (!root) return res;
        while (root) {
            if (root->right) {
                auto p = root->right;
                while (p->left && p->left != root) p = p->left;
                if (!p->left) {
                    res.push_back(root->val);
                    p->left = root;
                    root = root->right;
                } else {
                    p->left = NULL;
                    root = root->left;
                }
            } else {
                res.push_back(root->val);
                root = root->left;
            }
        }
        reverse(res.begin(), res.end());
        return res;
    }
};