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We can utilize the properties of a BST to perform a recursive traversal. The strategy here involves:
low, we need to trim the left subtree and consider the right subtree.high, we trim the right subtree and consider the left subtree.low, high], we recursively trim both subtrees.Time Complexity: O(n), where n is the number of nodes in the tree, since each node is processed once.
Space Complexity: O(h), where h is the height of the tree, representing the recursion stack.
1public class TreeNode {
2 public int val;
3 public TreeNode left;
4 public TreeNode right;
5 public TreeNode(int x) { val = x; }
6}
7
8public class Solution {
9 public TreeNode TrimBST(TreeNode root, int low, int high) {
10 if (root == null) return null;
11 if (root.val < low) return TrimBST(root.right, low, high);
12 if (root.val > high) return TrimBST(root.left, low, high);
13 root.left = TrimBST(root.left, low, high);
14 root.right = TrimBST(root.right, low, high);
15 return root;
16 }
17}The C# solution uses similar recursion logic to adjust and trim the BST based on the node values compared to the given bounds.
This iterative approach uses a stack to traverse the tree. The main idea is to mimic the recursive depth-first search using an explicit stack.
Time Complexity: O(n), as each node is processed once.
Space Complexity: O(h), where h is the height of the tree, due to the stack usage.
1#include <iostream>
2#include <stack>
3using namespace std;
struct TreeNode {
int val;
TreeNode *left;
TreeNode *right;
TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};
TreeNode* trimBST(TreeNode* root, int low, int high) {
if (!root) return NULL;
while (root && (root->val < low || root->val > high)) {
root = root->val < low ? root->right : root->left;
}
TreeNode* current = root;
stack<TreeNode*> nodeStack;
while (current || !nodeStack.empty()) {
while (current) {
if (current->val >= low && current->val <= high) {
nodeStack.push(current);
current = current->left;
} else {
current = current->val < low ? current->right : current->left;
}
}
if (!nodeStack.empty()) {
current = nodeStack.top()->right;
nodeStack.pop();
}
}
return root;
}Instead of using recursion, this C++ implementation uses a stack to perform depth-first search-like traversal. It adjusts the nodes based on their values compared to the bounds low and high, ensuring all nodes in the stack are within the bounds.