13. Pair Sum in BST

The problem can be found at the following link: Question Link

Problem Description

Given a Binary Search Tree (BST) and a target value, check whether there exists a pair of nodes in the BST whose sum equals the given target.

Examples

Example 1:

Input:

        7
       / \
      3   8
     / \    \
    2   4    9

Target = 12

Output:

True

Explanation:

In the given BST, there exist two nodes (8 and 4) that sum up to 12.

Example 2:

Input:

        9
       / \
      5   10
     / \     \
    2   6     12

Target = 23

Output:

False

Explanation:

No pair of nodes in the BST sum up to 23.

Constraints:

• $(1 \leq \text{Number of Nodes} \leq 10^5)$

• $(1 \leq \text{target} \leq 10^6)$

My Approach

Optimized Two-Pointer on Inorder Traversal (O(N) Time, O(N) Space)

1. Convert BST to sorted array using inorder traversal.

2. Use two-pointer approach (left and right) to find the target sum efficiently.

Algorithm Steps:

1. Perform an inorder traversal and store the BST elements in a sorted list.

2. Use two pointers (left at the start, right at the end) and check the sum:

   • If arr[left] + arr[right] == target, return true.

   • If arr[left] + arr[right] < target, move left++.

   • If arr[left] + arr[right] > target, move right--.

3. If no such pair exists, return false.

Time and Auxiliary Space Complexity

• Expected Time Complexity: O(N), since we traverse each node once.

• Expected Auxiliary Space Complexity: O(N), due to storing the inorder traversal.

Code (C++)

class Solution {
public:
    bool findTarget(Node* root, int target) {
        vector<int> inorder;
        function<void(Node*)> traverse = [&](Node* node) {
            if (!node) return;
            traverse(node->left);
            inorder.push_back(node->data);
            traverse(node->right);
        };
        traverse(root);
        int left = 0, right = inorder.size() - 1;
        while (left < right) {
            int sum = inorder[left] + inorder[right];
            if (sum == target) return true;
            sum < target ? left++ : right--;
        }
        return false;
    }
};

🌲 Alternative Approaches

2️⃣ Optimized Hash Set Approach (O(N) Time, O(H) Space)

1. Use DFS to traverse the tree while maintaining a hash set of visited values.

2. Check if (target - current node value) exists in the set, if yes, return true.

3. Insert the current node value into the set and continue.

class Solution {
public:
    bool findTarget(Node* root, int target) {
        unordered_set<int> seen;
        return dfs(root, target, seen);
    }

    bool dfs(Node* root, int target, unordered_set<int>& seen) {
        if (!root) return false;
        if (seen.count(target - root->data)) return true;
        seen.insert(root->data);
        return dfs(root->left, target, seen) || dfs(root->right, target, seen);
    }
};

🔹 Avoids extra space for storing inorder traversal 🔹 Uses Hash Set for O(1) lookup

3️⃣ BST Iterator + Two-Pointer (O(N) Time, O(H) Space)

1. Use two iterators: one for inorder (left to right) and one for reverse inorder (right to left).

2. Move iterators towards each other until the pair is found or iterators cross.

class BSTIterator {
    stack<Node*> st;
    bool reverse;
public:
    BSTIterator(Node* root, bool isReverse) {
        reverse = isReverse;
        pushAll(root);
    }

    int next() {
        Node* node = st.top(); st.pop();
        if (!reverse) pushAll(node->right);
        else pushAll(node->left);
        return node->data;
    }

    bool hasNext() { return !st.empty(); }

private:
    void pushAll(Node* node) {
        while (node) {
            st.push(node);
            node = reverse ? node->right : node->left;
        }
    }
};

class Solution {
public:
    bool findTarget(Node* root, int target) {
        BSTIterator l(root, false), r(root, true);
        int i = l.next(), j = r.next();
        while (i < j) {
            int sum = i + j;
            if (sum == target) return true;
            sum < target ? i = l.next() : j = r.next();
        }
        return false;
    }
};

🔹 Optimized space using O(H) stack instead of O(N) vector 🔹 Uses BST Iterators for an efficient two-pointer search

Comparison of Approaches

Approach	⏱️ Time Complexity	🗂️ Space Complexity	⚡ Method	✅ Pros	⚠️ Cons
Two-Pointer on Inorder Array	🟢 O(N)	🟡 O(N)	Two Pointers	Simple and efficient	Extra space for list
Hash Set (DFS)	🟢 O(N)	🟡 O(N)	Hashing	Faster lookup, no sorting needed	Hashing overhead
BST Iterator (Two-Pointer)	🟡 O(N^2)	🟢 O(H)	Recursion	No extra storage used	Slow for large BSTs

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💡 Best Choice?

• For simplicity: ✅ Two-Pointer on Inorder Array is easiest to understand.

• For faster lookups: ✅ Hash Set (O(H) space) avoids sorting overhead.

• For minimal space: ✅ BST Iterator (O(H) space, O(N) time) avoids extra space but is slow for large BSTs.

Code (Java)

class Solution {
    public boolean findTarget(Node root, int target) {
        List<Integer> inorder = new ArrayList<>();
        inorderTraversal(root, inorder);
        int left = 0, right = inorder.size() - 1;
        while (left < right) {
            int sum = inorder.get(left) + inorder.get(right);
            if (sum == target) return true;
            if (sum < target) left++;
            else right--;
        }
        return false;
    }

    private void inorderTraversal(Node node, List<Integer> inorder) {
        if (node == null) return;
        inorderTraversal(node.left, inorder);
        inorder.add(node.data);
        inorderTraversal(node.right, inorder);
    }
}

Code (Python)

class Solution:
    def findTarget(self, root, target):
        inorder = []
        def inorderTraversal(node):
            if not node:
                return
            inorderTraversal(node.left)
            inorder.append(node.data)
            inorderTraversal(node.right)

        inorderTraversal(root)
        left, right = 0, len(inorder) - 1
        while left < right:
            total = inorder[left] + inorder[right]
            if total == target:
                return True
            if total < target:
                left += 1
            else:
                right -= 1
        return False

Contribution and Support

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