114. 二叉树展开为链表

题解：特殊的后序遍历，右->左->中

https://leetcode.wang/leetcode-114-Flatten-Binary-Tree-to-Linked-List.html

/**
 *  * 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) {}
 *          * };
 *           */
class Solution {
public:
    void flatten(TreeNode* root) {
        TreeNode* pre = nullptr;
        back_order(root, pre);
    }
    void back_order(TreeNode* root, TreeNode*& pre) {
        if(root == nullptr) {
            return;
        }
        back_order(root->right, pre);
        back_order(root->left, pre);
        root->right = pre;
        root->left = nullptr;
        pre = root;
    }
};