# Day 23 二叉树 - 修剪二叉搜索树

## 669\. 修剪二叉搜索树

```typescript
function trimBST(root: TreeNode | null, low: number, high: number): TreeNode | null {
	if (root === null) {
		return null;
	}
	
	if (root.val > high) {
		return trimBST(root.left, low, high);
	}
	
	if (root.val < low) {
		return trimBST(root.right, low, high);
	}
	
	root.left = trimBST(root.left, low, high);
	root.right = trimBST(root.right, low, high);
	return root;
};
```

## 108\. 将有序数组转换为二叉搜索树

```typescript
function sortedArrayToBST(nums: number[]): TreeNode | null {
	const buildTree = (nums: number[], left: number , right: number) => {
		if (left > right) {
			return null;
		};
		
		let mid = Math.floor(left + (right - left) / 2);
		let root = new TreeNode(nums[mid]);
		root.left = buildTree(nums, left, mid - 1);
		root.right = buildTree(nums, mid + 1, right);
		return root;
	};
	return buildTree(nums, 0, nums.length - 1);
};
```

## 538\. 把二叉搜索树转换为累加树

```typescript
function convertBST(root: TreeNode | null): TreeNode | null {
	let sum = 0;
	
	const dfs = (node: TreeNode | null) => {
		if (node) {
			dfs(node.right);
			node.val += sum;
			sum = node.val;
			dfs(node.left);
		}
	}
	dfs(root);
	return root;
};
```