Three Sum Problem All Unique Triplets in DSA Rust - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to sort the input vector before processing.

DSA Rust

nums.[1]();

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Ashuffle

Breverse

Csort

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using reverse instead of sort.

Trying to shuffle the vector.

Removing elements instead of sorting.

Sorting the vector helps to efficiently find triplets and avoid duplicates.

2fill in blank

medium

Complete the code to skip duplicate elements for the first pointer i.

DSA Rust

if i > 0 && nums[i] == nums[i[1]] { continue; }

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A/2

B+1

C*2

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using i + 1 which looks ahead instead of behind.

Using multiplication or division which is incorrect here.

We compare the current element with the previous one to skip duplicates.

3fill in blank

hard

Fix the error in the while loop condition to move the right pointer correctly.

DSA Rust

while left < right && nums[right] [1] sum { right -= 1; }

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A<

B>

C==

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using less than instead of greater than.

Using equality which stops too early.

Using not equal which is incorrect here.

We move the right pointer leftwards while the value is greater than the target sum.

4fill in blank

hard

Fill both blanks to skip duplicates for left and right pointers after finding a valid triplet.

DSA Rust

while left < right && nums[left] == nums[left[1]] { left += 1; }
while left < right && nums[right] == nums[right[2]] { right -= 1; }

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A+1

B-1

C+2

D-2

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong direction for increment or decrement.

Skipping too many elements by using +2 or -2.

We move left forward and right backward to skip duplicates after adding a triplet.

5fill in blank

hard

Fill all three blanks to correctly add the found triplet to the result vector.

DSA Rust

result.push(vec![nums[[1]], nums[[2]], nums[[3]]]);

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Ai

Bleft

Cright

Dsum

Attempts:

3 left

💡 Hint

Common Mistakes

Using sum which is not an index.

Mixing up the order of pointers.

The triplet consists of elements at indices i, left, and right.

Practice

1. What is the main goal of the Three Sum Problem All Unique Triplets in data structures and algorithms?

easy

A. Find the maximum element in the array

B. Find all unique triplets in an array that sum to zero

C. Sort the array in ascending order

D. Count the number of pairs that sum to zero

Solution

1. Step 1: Understand the problem statement

   The problem asks to find all unique sets of three numbers in the array whose sum is zero.

2. Step 2: Clarify the output

   The output should be a list of triplets with no duplicates, each summing to zero.

3. Final Answer:

   Find all unique triplets in an array that sum to zero -> Option B

4. Quick Check:

   Three Sum Problem = Find unique triplets summing to zero [OK]

Hint: Look for three numbers adding to zero without duplicates [OK]

Common Mistakes:

• Confusing with pair sum problems
• Ignoring uniqueness of triplets
• Only finding one triplet instead of all

2. Which Rust code snippet correctly initializes a vector to solve the Three Sum problem?

easy

A. let nums = [1, -1, 0, 2, -2];

B. let nums = {1, -1, 0, 2, -2};

C. let nums = (1, -1, 0, 2, -2);

D. let nums = vec![1, -1, 0, 2, -2];

Solution

1. Step 1: Identify Rust vector syntax

   In Rust, vectors are created using vec![elements] syntax.

2. Step 2: Check each option

   let nums = vec![1, -1, 0, 2, -2]; uses vec![], which is correct. let nums = [1, -1, 0, 2, -2]; is an array, not a vector. let nums = (1, -1, 0, 2, -2); is a tuple. let nums = {1, -1, 0, 2, -2}; is invalid syntax.

3. Final Answer:

   let nums = vec![1, -1, 0, 2, -2]; -> Option D

4. Quick Check:

   Vector initialization uses vec! macro [OK]

Hint: Use vec![] to create vectors in Rust [OK]

Common Mistakes:

• Using array syntax instead of vector
• Using tuple instead of vector
• Invalid braces for initialization

3. What is the output of the following Rust code snippet that finds unique triplets summing to zero?

fn main() {
    let mut nums = vec![-1, 0, 1, 2, -1, -4];
    nums.sort();
    let mut result = Vec::new();
    for i in 0..nums.len() - 2 {
        if i > 0 && nums[i] == nums[i - 1] { continue; }
        let (mut left, mut right) = (i + 1, nums.len() - 1);
        while left < right {
            let sum = nums[i] + nums[left] + nums[right];
            if sum == 0 {
                result.push(vec![nums[i], nums[left], nums[right]]);
                while left < right && nums[left] == nums[left + 1] { left += 1; }
                while left < right && nums[right] == nums[right - 1] { right -= 1; }
                left += 1;
                right -= 1;
            } else if sum < 0 {
                left += 1;
            } else {
                right -= 1;
            }
        }
    }
    println!("{:?}", result);
}

medium

A. [[-1, -1, 2], [-1, 0, 1]]

B. [[-4, 0, 4], [-1, -1, 2]]

C. [[0, 0, 0]]

D. []

Solution

1. Step 1: Sort the array

   Sorted array is [-4, -1, -1, 0, 1, 2].

2. Step 2: Find triplets summing to zero

   Triplets found are [-1, -1, 2] and [-1, 0, 1].

3. Final Answer:

   [[-1, -1, 2], [-1, 0, 1]] -> Option A

4. Quick Check:

   Unique triplets summing zero = [[-1,-1,2],[-1,0,1]] [OK]

Hint: Sort array, use two pointers to find triplets [OK]

Common Mistakes:

• Not sorting before two-pointer approach
• Including duplicate triplets
• Incorrect pointer movement

4. Identify the bug in this Rust code snippet for the Three Sum problem:

fn three_sum(nums: &mut Vec<i32>) -> Vec {
    nums.sort();
    let mut res = Vec::new();
    for i in 0..nums.len() {
        if i > 0 && nums[i] == nums[i - 1] { continue; }
        let (mut left, mut right) = (i + 1, nums.len() - 1);
        while left < right {
            let sum = nums[i] + nums[left] + nums[right];
            if sum == 0 {
                res.push(vec![nums[i], nums[left], nums[right]]);
                left += 1;
                right -= 1;
            } else if sum < 0 {
                left += 1;
            } else {
                right -= 1;
            }
        }
    }
    res
}

medium

A. Loop runs beyond valid range causing panic

B. Sorting is done after the loop

C. Missing duplicate skipping after finding a triplet

D. Using immutable vector instead of mutable

Solution

1. Step 1: Check duplicate handling after triplet found

   The code does not skip duplicates for left and right pointers after pushing a triplet.

2. Step 2: Understand impact

   Without skipping duplicates, duplicate triplets may be added to the result.

3. Final Answer:

   Missing duplicate skipping after finding a triplet -> Option C

4. Quick Check:

   Duplicate skipping after triplet push is required [OK]

Hint: Skip duplicates after adding triplet to avoid repeats [OK]

Common Mistakes:

• Not skipping duplicates for left and right pointers
• Incorrect loop range causing panic
• Sorting after loop instead of before

5. Given the input vector vec![0, 0, 0, 0], what is the correct output of a properly implemented Three Sum solution that finds all unique triplets summing to zero?

hard

A. [[0, 0, 0]]

B. [[0, 0, 0], [0, 0, 0]]

C. []

D. [[0, 0]]

Solution

1. Step 1: Analyze input with duplicates

   Input has four zeros, so multiple triplets of zeros exist but are duplicates.

2. Step 2: Unique triplets only

   The solution must return only one unique triplet [0, 0, 0].

3. Final Answer:

   [[0, 0, 0]] -> Option A

4. Quick Check:

   Only one unique zero triplet exists [OK]

Hint: Return unique triplets only, even if duplicates exist [OK]

Common Mistakes:

• Returning duplicate triplets multiple times
• Returning empty list incorrectly
• Returning pairs instead of triplets