Trim a Binary Search Tree - Recursive C++ solution

Binary Search Tree     Binary Tree     C++     Depth-First Search     Medium     Tree    

Problem Statement:

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]

 

Constraints:

• The number of nodes in the tree is 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

Solution:

The important observation is that: If parent<low => parent->left < low Similarly If parent>high => parent->right> high Thus we can reject the entire subtree depending on this condition.

We can build the tree recursively and return root.

 
class Solution {
public:
    TreeNode* trimBST(TreeNode* root, int low, int high) {
        if (root==NULL) return root;
        if (root->val < low) return trimBST(root->right, low, high);
        if (root->val > high) return trimBST(root->left, low, high);
        root->left = trimBST(root->left, low, high);
        root->right = trimBST(root->right, low, high);
        return root;
    }
};