本代码基于树的中序遍历非递归版

class Solution {
    public TreeNode convert(TreeNode root) {
        if(root==null) return null;
        Stack<TreeNode> stack=new Stack();
        List<TreeNode> list=new LinkedList();
        TreeNode cur=root;
        while(!stack.empty() || cur!=null){
            if(cur!=null){
                stack.push(cur);
                cur=cur.left;
            }else{
                TreeNode node=stack.pop();
                int size=list.size();
                if(0==size){
                    node.left=null;
                }else{
                    TreeNode preNode=list.get(size-1);
                    preNode.right=node;
                    node.left=preNode;
                }
                list.add(node);
                cur=node.right;
            }
        }
        list.get(list.size()-1).right=null;
        return list.get(0);
    }
}