Increasing Order Search Tree

# 897. Increasing Order Search Tree

Given the root of a binary search tree, rearrange the tree in in-order so that the leftmost node in the tree is now the root of the tree, and every node has no left child and only one right child.

### Example 1:
```
Input: root = [5,3,6,2,4,null,8,1,null,null,null,7,9]
Output: [1,null,2,null,3,null,4,null,5,null,6,null,7,null,8,null,9]
```

### Example 2:
```
Input: root = [5,1,7]
Output: [1,null,5,null,7]
 ```

Constraints:
```
The number of nodes in the given tree will be in the range [1, 100].
0 <= Node.val <= 1000
```

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
 var increasingBST = function(root) {
    
    const out = new TreeNode();
    let cur = out; 

    const inOrderTraverse = (node) => {
        if (!node) {
            return;
        }

        inOrderTraverse(node.left);

        node.left = null;
        cur.right = node;
        cur = node;
        inOrderTraverse(node.right);
    }

    inOrderTraverse(root);
    return out.right;
};