sword to offer 026

struct TreeNode {
	int val;
	struct TreeNode *left;
	struct TreeNode *right;
	TreeNode(int x) :
			val(x), left(NULL), right(NULL) {
	}
};*/
class  {
private:
    TreeNode* pre = nullptr;
    TreeNode* res = nullptr;
    void Dfs(TreeNode* root) {
        if(root == nullptr) {
            return ;
        }
        Dfs(root->left);
        if(pre == nullptr) {
            pre = root;
            root->left = nullptr;
            res = pre;
        } else {
            root->left = pre;
            pre->right = root;
            pre = root;
        }
        Dfs(root->right);
    }
public:
    TreeNode* Convert(TreeNode* pRootOfTree) {
        Dfs(pRootOfTree);
        return res;
    }
};