https://leetcode-cn.com/problems/increasing-order-search-tree/
class Solution { TreeNode pre; public TreeNode increasingBST(TreeNode root) { TreeNode tmp=new TreeNode(-1); pre=tmp; dfs(root); return tmp.right; } public void dfs(TreeNode root){ if(root==null){ return; } dfs(root.left); pre.right=root; root.left=null; pre=root; dfs(root.right); } }