递增顺序查找树

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给定一个树，按中序遍历重新排列树，使树中最左边的结点现在是树的根，并且每个结点没有左子结点，只有一个右子结点。
示例 ：

输入：[5,3,6,2,4,null,8,1,null,null,null,7,9]

   5
  / \
3    6

/ \ \
2 4 8
/ / \
1 7 9

输出：[1,null,2,null,3,null,4,null,5,null,6,null,7,null,8,null,9]

1
\
2
\
3
\
4
\
5
\
6
\
7
\
8
\
9 

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/**
 * 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:
    
    
    TreeNode* increasingBST(TreeNode* root) {
        if (root == NULL) return root;
        root->right = increasingBST(root->right);
        if (root->left) {
            TreeNode* left = root->left;
            root->left = NULL;
            TreeNode* node = left;
            while (node->right) {
                node = node->right;
            }
            node->right = root;
            return increasingBST(left);
         }
        return root;
        
    }
};