# 10.Trim a Binary Search Tree

Given the `root` of a binary search tree and the lowest and highest boundaries as `low` and `high`, trim the tree so that all its elements lies in `[low, high]`. Trimming the tree should **not** change the relative structure of the elements that will remain in the tree (i.e., any node's descendant should remain a descendant). It can be proven that there is a **unique answer**.

Return *the root of the trimmed binary search tree*. Note that the root may change depending on the given bounds.

```
Example 1:
Input: root = [1,0,2], low = 1, high = 2
Output: [1,null,2]

Example 2:
Input: root = [3,0,4,null,2,null,null,1], low = 1, high = 3
Output: [3,2,null,1]
```

## Solution : 

**Approach**\
**Removing nodes from bottom using Postorder Traversal** 

```cpp
class Solution
{
public:
    TreeNode *trimBST(TreeNode *root, int low, int high)
    {

        if (root == NULL)
        {
            return NULL;
        }
        
        
        root->left = trimBST(root->left, low, high);
        root->right = trimBST(root->right, low, high);

        if (root->val < low)
        {
            return root->right;
        }

        if (root->val > high)
        {
            return root->left;
        }

        return root;
    }
};
```

**Time Complexity: O(n)**