java容易理解

public class Solution {
    public TreeNode Convert(TreeNode root) {
        if(root == null){
            return null;
        }
        TreeNode newHead = root;
        TreeNode pre;
        if(root.left != null){ //排序左边
             newHead = Convert(root.left);
             pre = newHead;
            while(pre.right != null){
                pre = pre.right;
            }
            pre.right = root;
            root.left = pre;
        }
        if(root.right != null){//排序右边
            pre = Convert(root.right);
            pre.left = root;
            root.right = pre;
        }
        return newHead;
    }
}