0669. Trim a Binary Search Tree

https://leetcode.com/problems/trim-a-binary-search-tree

Description

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]

Example 3:

**Input:** root = [1], low = 1, high = 2
**Output:** [1]

Example 4:

**Input:** root = [1,null,2], low = 1, high = 3
**Output:** [1,null,2]

Example 5:

**Input:** root = [1,null,2], low = 2, high = 4
**Output:** [2]

Constraints:

• The number of nodes in the tree in the range [1, 104].

• 0 <= Node.val <= 104

• The value of each node in the tree is unique.

• root is guaranteed to be a valid binary search tree.

• 0 <= low <= high <= 104

ac

class Solution {
    public TreeNode trimBST(TreeNode root, int L, int R) {
        if (root == null) return root;

        if (root.val < L) return trimBST(root.right, L, R);
        else if (root.val > R) return trimBST(root.left, L, R);

        root.left = trimBST(root.left, L, R);
        root.right = trimBST(root.right, L, R);

        return root;
    }
}

/*
recursively process left/right
*/