> For the complete documentation index, see [llms.txt](https://junnie.gitbook.io/nine-chapter/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://junnie.gitbook.io/nine-chapter/2.binary-tree/453.flatten-binary-tree-to-linked-list.md).

# 453.Flatten Binary Tree to Linked List

## 1.Description(Easy)

Flatten a binary tree to a fake "linked list" in pre-order traversal.

Here we use the\_right\_pointer in TreeNode as the \_next \_pointer in ListNode.

### Notice

Don't forget to mark the left child of each node to null. Or you will get Time Limit Exceeded or Memory Limit Exceeded.

**Example**

```
              1
               \
     1          2
    / \          \
   2   5          3
  / \   \          \
 3   4   6          4
                     \
                      5
                       \
                        6
```

## 2.Code

用一个栈存放顺序每次放进去就peek出来，再依次放出来连接。别忘了把left设置成null.如果是最后把right设置成null.

```
public void flatten(TreeNode root){
    if(root==null){
        return;
    }
    Stack<TreeNode> st=new Stack<TreeNode>();
    st.push(root);
    while(!st.empty()){
        TreeNode node=st.pop();
        if(node.right!=null){
            st.push(node.right);
        }
        if(node.left!=null){
            st.push(node.left);
        }

        //connect
        node.left=null; //every node left is null;
        if(!st.empty()){
            node.right=st.peek(); //only peek.
        }else{
            node.right=null;
        }
    }
}
```


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