Two Sum in BST in DSA Go - Build from Scratch

Two Sum in BST

📖 Scenario: You are working with a phone book stored as a Binary Search Tree (BST). Each node contains a phone number. You want to find if there are two phone numbers in the BST that add up to a specific target sum.

🎯 Goal: Build a program that creates a BST with given phone numbers, sets a target sum, checks if any two numbers in the BST add up to the target, and prints the result.

📋 What You'll Learn

Create a BST with exact phone numbers: 10, 5, 15, 3, 7, 18

Create a variable called target with value 22

Write a function findTarget that returns true if two numbers in the BST add up to target

Print true or false depending on the result

💡 Why This Matters

🌍 Real World

Finding pairs of numbers that sum to a target is useful in financial applications, phone number analysis, and data validation.

💼 Career

This problem helps prepare for coding interviews and teaches tree traversal and hash map usage, common in software engineering roles.

1

Create the BST nodes

Create a struct type called TreeNode with fields Val int, Left *TreeNode, and Right *TreeNode. Then create the BST root node called root with value 10, and add nodes with values 5, 15, 3, 7, and 18 as shown in the example.

DSA Go

type TreeNode struct {
    Val   int
    Left  *TreeNode
    Right *TreeNode
}

// Create the BST nodes below

Hint

Define the TreeNode struct first. Then create the root node and link child nodes to build the BST.

2

Set the target sum

Create an integer variable called target and set it to 22.

DSA Go

package main

import "fmt"

type TreeNode struct {
    Val   int
    Left  *TreeNode
    Right *TreeNode
}

func main() {
    root := &TreeNode{Val: 10}
    root.Left = &TreeNode{Val: 5}
    root.Right = &TreeNode{Val: 15}
    root.Left.Left = &TreeNode{Val: 3}
    root.Left.Right = &TreeNode{Val: 7}
    root.Right.Right = &TreeNode{Val: 18}

    // Create the target variable below
    // Your code here
}

Hint

Just write target := 22 inside the main function.

3

Write the function to find two sum in BST

Write a function called findTarget that takes root *TreeNode and k int as parameters and returns bool. Use a map to store visited values and check if k - current node value exists. Traverse the BST recursively.

DSA Go

package main

import "fmt"

type TreeNode struct {
    Val   int
    Left  *TreeNode
    Right *TreeNode
}

func main() {
    root := &TreeNode{Val: 10}
    root.Left = &TreeNode{Val: 5}
    root.Right = &TreeNode{Val: 15}
    root.Left.Left = &TreeNode{Val: 3}
    root.Left.Right = &TreeNode{Val: 7}
    root.Right.Right = &TreeNode{Val: 18}

    target := 22

    // Write the findTarget function call below
    // Your code here
}

// Write the findTarget function below

Hint

Use a helper function dfs inside findTarget to traverse the tree recursively. Use a map to check if the complement exists.

4

Print the result

Print the value of result using fmt.Println(result).

DSA Go

package main

import "fmt"

type TreeNode struct {
    Val   int
    Left  *TreeNode
    Right *TreeNode
}

func findTarget(root *TreeNode, k int) bool {
    visited := make(map[int]bool)
    var dfs func(node *TreeNode) bool
    dfs = func(node *TreeNode) bool {
        if node == nil {
            return false
        }
        if visited[k-node.Val] {
            return true
        }
        visited[node.Val] = true
        return dfs(node.Left) || dfs(node.Right)
    }
    return dfs(root)
}

func main() {
    root := &TreeNode{Val: 10}
    root.Left = &TreeNode{Val: 5}
    root.Right = &TreeNode{Val: 15}
    root.Left.Left = &TreeNode{Val: 3}
    root.Left.Right = &TreeNode{Val: 7}
    root.Right.Right = &TreeNode{Val: 18}

    target := 22

    result := findTarget(root, target)

    // Print the result below
    // Your code here
}

Hint

Use fmt.Println(result) to print the boolean result.

Practice

1. What is the main goal of the Two Sum in BST problem?

easy

A. Find the maximum value in the BST.

B. Find two nodes in the BST whose values add up to a given target sum.

C. Count the total number of nodes in the BST.

D. Check if the BST is balanced.

Solution

1. Step 1: Understand the problem statement

   The problem asks to find two nodes in a Binary Search Tree (BST) such that their values add up to a given target sum.

2. Step 2: Identify the goal

   The goal is to check if such a pair exists, not to find max, count nodes, or check balance.

3. Final Answer:

   Find two nodes in the BST whose values add up to a given target sum. -> Option B

4. Quick Check:

   Two Sum in BST = Find pair with target sum [OK]

Hint: Look for two values that add up to target in BST [OK]

Common Mistakes:

• Confusing with finding max or min value
• Trying to count nodes instead of finding pairs
• Checking BST balance instead of sum pairs

2. Which of the following Go function signatures is correct for a function that checks if two sum exists in a BST?

easy

A. func findTarget(root *TreeNode, k int) bool

B. func findTarget(root TreeNode, k string) int

C. func findTarget(root *TreeNode, k float64) string

D. func findTarget(root TreeNode, k bool) *TreeNode

Solution

1. Step 1: Understand function parameters and return type

   The function should take a pointer to the root node of BST and an integer target sum, returning a boolean indicating if pair exists.

2. Step 2: Match correct signature

   func findTarget(root *TreeNode, k int) bool matches: pointer to TreeNode, integer target, returns bool.

3. Final Answer:

   func findTarget(root *TreeNode, k int) bool -> Option A

4. Quick Check:

   Correct function signature = func findTarget(root *TreeNode, k int) bool [OK]

Hint: Function takes *TreeNode and int, returns bool [OK]

Common Mistakes:

• Using wrong parameter types like string or bool
• Passing TreeNode by value instead of pointer
• Returning wrong type like int or string

3. What will be the output of the following Go code snippet for Two Sum in BST?

type TreeNode struct {
    Val int
    Left, Right *TreeNode
}

func findTarget(root *TreeNode, k int) bool {
    seen := make(map[int]bool)
    var dfs func(*TreeNode) bool
    dfs = func(node *TreeNode) bool {
        if node == nil {
            return false
        }
        if seen[k - node.Val] {
            return true
        }
        seen[node.Val] = true
        return dfs(node.Left) || dfs(node.Right)
    }
    return dfs(root)
}

root := &TreeNode{Val:5, Left:&TreeNode{Val:3}, Right:&TreeNode{Val:6}}
result := findTarget(root, 9)
fmt.Println(result)

medium

A. true

B. false

C. Compilation error

D. Runtime panic

Solution

1. Step 1: Trace the DFS traversal and map usage

   The function uses DFS to traverse nodes. It checks if (target - current node value) is in the map. For root 5, seen is empty, adds 5. Goes left to 3, checks if 9-3=6 in seen (no), adds 3. Goes right to 6, checks if 9-6=3 in seen (yes), returns true.

2. Step 2: Confirm output

   Since pair (3,6) sums to 9, function returns true and prints true.

3. Final Answer:

   true -> Option A

4. Quick Check:

   Pair found (3+6=9) = true [OK]

Hint: Check if complement exists in map during DFS [OK]

Common Mistakes:

• Not checking complement before adding current value
• Returning false too early
• Ignoring nil node checks causing panic

4. Identify the bug in this Go code for Two Sum in BST and choose the fix:

func findTarget(root *TreeNode, k int) bool {
    seen := make(map[int]bool)
    var dfs func(*TreeNode) bool
    dfs = func(node *TreeNode) bool {
        if node == nil {
            return false
        }
        seen[node.Val] = true
        if seen[k - node.Val] {
            return true
        }
        return dfs(node.Left) || dfs(node.Right)
    }
    return dfs(root)
}

medium

A. Add a nil check after adding to map.

B. Remove the map and use a slice instead.

C. Change return type to int instead of bool.

D. Check complement before adding current node value to map.

Solution

1. Step 1: Analyze the order of operations in DFS

   The code adds current node value to map before checking if complement exists. This causes the complement check to always find current value, leading to false positives.

2. Step 2: Correct the order

   Check if complement exists in map before adding current node value to avoid matching the same node twice.

3. Final Answer:

   Check complement before adding current node value to map. -> Option D

4. Quick Check:

   Complement check before map insert fixes bug [OK]

Hint: Check complement before adding current value [OK]

Common Mistakes:

• Adding current value before checking complement
• Using wrong data structure for lookup
• Ignoring nil node checks

5. Given a BST with nodes [4, 2, 6, 1, 3, 5, 7], which pairs sum to 9? Select the correct output of a Two Sum in BST function that returns all pairs.

func findAllPairs(root *TreeNode, k int) [][2]int {
    seen := make(map[int]bool)
    pairs := [][2]int{}
    var dfs func(*TreeNode)
    dfs = func(node *TreeNode) {
        if node == nil {
            return
        }
        complement := k - node.Val
        if seen[complement] {
            pairs = append(pairs, [2]int{complement, node.Val})
        }
        seen[node.Val] = true
        dfs(node.Left)
        dfs(node.Right)
    }
    dfs(root)
    return pairs
}

hard

A. [[5,4],[6,3],[7,2]]

B. [[1,8],[2,7],[3,6]]

C. [[3,6],[4,5],[2,7]]

D. [[1,7],[2,6],[3,5]]

Solution

1. Step 1: Identify all pairs in BST that sum to 9

   Nodes are 1,2,3,4,5,6,7. Pairs summing to 9 are (2,7), (3,6), (4,5).

2. Step 2: Match pairs with function output format

   The function appends pairs as [complement, node.Val] during preorder traversal: at 6 finds 3 in seen, appends [3,6]; at 5 finds 4, [4,5]; at 7 finds 2, [2,7].

3. Final Answer:

   [[3,6],[4,5],[2,7]] -> Option C

4. Quick Check:

   Pairs summing to 9 in traversal order = [[3,6],[4,5],[2,7]] [OK]

Hint: List pairs where values sum to target k [OK]

Common Mistakes:

• Including pairs with values not in BST
• Reversing pair order incorrectly
• Missing valid pairs