# 1. Topic

Input a [binary search tree] , convert the binary search tree into a sorted doubly linked list.

# 2. Solution

## 2.1 The structure of the tree

```class TreeNode {
int val = 0;
TreeNode left = null;
TreeNode right = null;
public TreeNode(int val) {
this.val = val;
}
}```

## 2.2 Solution 1

The characteristic of a binary search tree is that the left is small and the right is large.
Consider, the return should be the leftmost child of this tree. i.e. the minimum value.
So find the node first.
And use in-order traversal to sort out the relationship.
Save the left subtree, the most recently visited value, through pre.

```public  class  Solution {
public TreeNode Convert ( TreeNode pRootOfTree ) {
if (pRootOfTree== null ) return  null ;
TreeNode left = pRootOfTree;
while (left.left!=null){
left = left.left;
}
helper(pRootOfTree);
return left;
}

TreeNode pre = null;
public void helper(TreeNode root) {
if (root == null) return ;
helper(root.left);
root.left = pre;
if(pre!=null) pre.right = root;
pre = root;
helper(root.right);
}
}```

# Summarize

Consider, the return should be the leftmost child of this tree. i.e. the minimum value.

The algorithm series has an open source project on github, mainly the demo code of this series of blogs. [https://github.com/forestnlp/alg]
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