114. Flatten Binary Tree to Linked List 将二叉树转换成链表

解题思路：

1）找到根节点的左子树的最右节点，然后将根节点的右子树接到前节点的右子树上，不断执行这个过程。

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void flatten(TreeNode* root) {
        if (root == nullptr) {
            return;
        }
        TreeNode *cur = root;
        while (cur) {
            if (cur->left) {               // 找到左子树的最后节点作为前节点
                TreeNode *pre = cur->left;
                while (pre->right) {
                    pre = pre->right;
                }
                pre->right = cur->right;
                cur->right = cur->left;
                cur->left = nullptr;
            }
            cur = cur->right;
        }
    }
};