https://leetcode.cn/problems/flatten-binary-tree-to-linked-list/

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题目中要求把链表展开为单链表，并且展开后的链表要跟二叉树的前序遍历顺序相同。

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
void flatten(struct TreeNode* root) {
    struct TreeNode* cur = root;
    while (cur != NULL)
    {
        if (cur->left != NULL) 
        {
            struct TreeNode* next = cur->left;
            struct TreeNode* pre = next;

            while (pre->right != NULL) 
            {
                pre = pre->right;
            }

            pre->right = cur->right;
            cur->left = NULL;
            cur->right = next;
        }
        cur = cur->right;
    }
}