Generate All Combinations Sum K in DSA Go - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start the backtracking with an empty combination.

DSA Go

func combinationSumK(candidates []int, k int) [][]int {
    var result [][]int
    var comb []int
    backtrack(candidates, k, 0, comb, &result)
    return result
}

func backtrack(candidates []int, target int, start int, comb []int, result *[][]int) {
    if target == 0 {
        *result = append(*result, append([]int{}, comb...))
        return
    }
    for i := start; i < len(candidates); i++ {
        if candidates[i] > target {
            continue
        }
        comb = append(comb, candidates[i])
        backtrack(candidates, target[1], i+1, comb, result)
        comb = comb[:len(comb)-1]
    }
}

Drag options to blanks, or click blank then click option'

A+ candidates[i]

B- candidates[i]

C* candidates[i]

D/ candidates[i]

Attempts:

3 left

💡 Hint

Common Mistakes

Adding the candidate to the target instead of subtracting.

Multiplying or dividing the target by the candidate.

We subtract the current candidate from the target to reduce the remaining sum needed.

2fill in blank

medium

Complete the code to avoid reusing the same candidate index in combinations.

DSA Go

func backtrack(candidates []int, target int, start int, comb []int, result *[][]int) {
    if target == 0 {
        *result = append(*result, append([]int{}, comb...))
        return
    }
    for i := start; i < len(candidates); i++ {
        if candidates[i] > target {
            continue
        }
        comb = append(comb, candidates[i])
        backtrack(candidates, target-candidates[i], [1], comb, result)
        comb = comb[:len(comb)-1]
    }
}

Drag options to blanks, or click blank then click option'

Ai+1

Bi

Cstart

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same index repeatedly causing infinite loops.

Resetting start to 0 causing duplicates.

We move to the next index (i+1) to avoid reusing the same candidate in the current combination.

3fill in blank

hard

Fix the error in the base case to correctly add a copy of the current combination to the result.

DSA Go

if target == 0 {
    *result = append(*result, [1])
    return
}

Drag options to blanks, or click blank then click option'

Aappend([]int{}, comb...)

Bcomb

Ccomb[:]

Dcopy(comb)

Attempts:

3 left

💡 Hint

Common Mistakes

Appending comb directly causing all results to be the same.

Using copy() which returns number of elements copied, not a slice.

We must append a copy of comb to avoid later modifications affecting stored combinations.

4fill in blank

hard

Fill both blanks to skip candidates that are greater than the remaining target.

DSA Go

for i := start; i < len(candidates); i++ {
    if candidates[i] [1] target {
        [2]
    }
    comb = append(comb, candidates[i])
    backtrack(candidates, target-candidates[i], i+1, comb, result)
    comb = comb[:len(comb)-1]
}

Drag options to blanks, or click blank then click option'

A>

B<

Ccontinue

Dbreak

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' causing wrong skips.

Using 'break' which stops the loop prematurely.

If candidate is greater than target, we skip it with continue to try next candidate.

5fill in blank

hard

Fill all three blanks to define the function signature and initialize variables correctly.

DSA Go

func [1](candidates []int, k int) [][]int {
    var [2] [][]int
    var comb []int
    backtrack(candidates, k, 0, comb, &[3])
    return [2]
}

Drag options to blanks, or click blank then click option'

AcombinationSumK

Bresult

Dcombinations

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatching variable names causing compilation errors.

Not passing result by reference.

Function is named combinationSumK; result slice is named result and passed by reference.

Practice

1. What does the problem "Generate All Combinations Sum K" ask you to find?

easy

A. All unique combinations of numbers that add up to K

B. The largest number less than K

C. The sum of all numbers from 1 to K

D. The product of all numbers that divide K

Solution

1. Step 1: Understand the problem goal

   The problem asks to find all unique sets of numbers that add exactly to the target sum K.

2. Step 2: Clarify what combinations mean

   Combinations mean groups of numbers where order does not matter, and each number can be used once or multiple times depending on problem rules.

3. Final Answer:

   All unique combinations of numbers that add up to K -> Option A

4. Quick Check:

   Combinations sum K = unique sets adding to K [OK]

Hint: Focus on sums that exactly equal K, ignoring order [OK]

Common Mistakes:

• Confusing combinations with permutations
• Looking for a single sum instead of all combinations
• Ignoring the target sum K

2. Which of the following Go function signatures is correct for a function that generates all combinations summing to K from a slice of integers?

easy

A. func combinationSum(nums []int, target int) int

B. func combinationSum(nums int, target []int) []int

C. func combinationSum(nums []int, target int) []int

D. func combinationSum(nums []int, target int) [][]int

Solution

1. Step 1: Identify input parameters

   The function should take a slice of integers and a target integer sum.

2. Step 2: Identify return type

   It should return a slice of slices of integers, representing all combinations.

3. Final Answer:

   func combinationSum(nums []int, target int) [][]int -> Option D

4. Quick Check:

   Input slice and target int, output slice of int slices [OK]

Hint: Return type must hold multiple combinations ([][]int) [OK]

Common Mistakes:

• Swapping parameter types
• Returning a single slice instead of slice of slices
• Returning an int instead of combinations

3. What is the output of the following Go code snippet?

func main() {
    nums := []int{2, 3, 6, 7}
    target := 7
    result := combinationSum(nums, target)
    fmt.Println(result)
}

func combinationSum(nums []int, target int) [][]int {
    var res [][]int
    var backtrack func(start, target int, path []int)
    backtrack = func(start, target int, path []int) {
        if target == 0 {
            temp := make([]int, len(path))
            copy(temp, path)
            res = append(res, temp)
            return
        }
        for i := start; i < len(nums); i++ {
            if nums[i] > target {
                continue
            }
            backtrack(i, target-nums[i], append(path, nums[i]))
        }
    }
    backtrack(0, target, []int{})
    return res
}

medium

A. [[2 2 3] [7]]

B. [[2 3 6] [7]]

C. [[3 3 3] [7]]

D. [[2 2 2 3] [7]]

Solution

1. Step 1: Understand backtracking logic

   The function tries combinations starting from each index, allowing reuse of numbers, and collects paths summing to target.

2. Step 2: Trace combinations for target 7

   Possible sums are [2,2,2,3] and [7] because 2+2+2+3=9 is incorrect, so check carefully: 2+2+2+3=9 (too big), so that is invalid. The actual combinations are [2,2,3] (2+2+3=7) and [7].

3. Step 3: Verify options

   [[2 2 2 3] [7]] shows [2 2 2 3] which sums to 9, invalid. [[2 2 3] [7]] shows [2 2 3] and [7], which is correct.

4. Final Answer:

   [[2 2 3] [7]] -> Option A

5. Quick Check:

   Backtracking finds [2 2 3] and [7] sums [OK]

Hint: Look for combinations that sum exactly to 7 using given nums [OK]

Common Mistakes:

• Including invalid sums like [2 3 6]
• Ignoring reuse of numbers
• Confusing order of elements

4. Identify the bug in this Go function for generating combinations summing to K:

func combinationSum(nums []int, target int) [][]int {
    var res [][]int
    var backtrack func(start, target int, path []int)
    backtrack = func(start, target int, path []int) {
        if target == 0 {
            res = append(res, path)
            return
        }
        for i := start; i < len(nums); i++ {
            if nums[i] > target {
                continue
            }
            backtrack(i, target-nums[i], append(path, nums[i]))
        }
    }
    backtrack(0, target, []int{})
    return res
}

medium

A. The target check should be target < 0 instead of target == 0

B. The loop should start from 0 every time, not from start

C. Appending path directly causes shared slice reference issues

D. The function should return a single slice, not slice of slices

Solution

1. Step 1: Check how path is appended to result

   Appending path directly stores the reference, causing all results to point to the same underlying array.

2. Step 2: Fix by copying path before appending

   Make a copy of path slice before appending to res to avoid shared references.

3. Final Answer:

   Appending path directly causes shared slice reference issues -> Option C

4. Quick Check:

   Copy path before append to avoid shared slice bugs [OK]

Hint: Always copy slices before appending to result [OK]

Common Mistakes:

• Appending slice references directly
• Incorrect loop start index
• Wrong base case condition

5. Given a list of integers with duplicates, how would you modify the combination sum algorithm to generate all unique combinations summing to K without duplicates? Choose the best approach.

hard

A. Use a map to store sums and ignore duplicates after generation

B. Sort the list and skip duplicate numbers during recursion

C. Allow duplicates freely and filter results at the end

D. Increase target by 1 to avoid duplicates

Solution

1. Step 1: Sort the input list

   Sorting helps group duplicates together so they can be skipped during recursion.

2. Step 2: Skip duplicates in recursion

   When iterating, skip numbers equal to the previous one at the same recursion level to avoid duplicate combinations.

3. Final Answer:

   Sort the list and skip duplicate numbers during recursion -> Option B

4. Quick Check:

   Sorting + skipping duplicates = unique combinations [OK]

Hint: Sort input and skip duplicates while recursing [OK]

Common Mistakes:

• Filtering duplicates after generating all combinations
• Not sorting input before recursion
• Changing target incorrectly