Is 3Sum Smaller easy or hard?

3Sum Smaller is generally considered a medium difficulty problem. The brute force idea is simple, but recognizing that sorting allows counting multiple valid pairs with two pointers is the key insight that reduces the complexity to O(n^2).

3Sum Smaller Python/Java solution

The typical implementation sorts the array and then iterates with an index i while using left and right pointers for the remaining elements. When the sum is less than the target, add right - left to the count and move the left pointer. The same logic works across Python, Java, C++, Go, TypeScript, and JavaScript.

How to solve 3Sum Smaller in O(n)?

Solving 3Sum Smaller in strict O(n) time is not possible for the general case because the algorithm must consider combinations of three elements. The best known practical solution is O(n^2) using sorting and the two pointers technique. This approach efficiently counts multiple valid triplets in a single step.

What is the best approach for 3Sum Smaller?

The most efficient approach sorts the array and then uses a two pointers technique. Fix the first element and move two pointers across the remaining subarray to count valid pairs. Because the array is sorted, when a sum is smaller than the target you can add multiple pairs at once. This reduces the complexity to O(n^2) time with O(1) extra space.

Is 3Sum Smaller asked at Google/Amazon/Meta?

3Sum-style problems frequently appear in interviews at companies like Google, Amazon, and Meta because they test sorting, two pointers, and combination counting. Variants include 3Sum, 3Sum Closest, and 3Sum Smaller. Interviewers expect candidates to move from brute force to an O(n^2) optimized solution.

What data structure is used in 3Sum Smaller?

The problem mainly relies on arrays combined with sorting and the two pointers technique. No complex data structures are required. The algorithm uses pointer movement over a sorted array to efficiently count valid triplets.

What is the time complexity of 3Sum Smaller?

The optimal solution runs in O(n^2) time after sorting the array. Sorting takes O(n log n), but the dominant cost is the quadratic two-pointer traversal for each first element. Space complexity is O(1) if sorting is done in place.

259. 3Sum Smaller - Practice Online

Given an array of n integers nums and an integer target, find the number of index triplets i, j, k with 0 <= i < j < k < n that satisfy the condition nums[i] + nums[j] + nums[k] < target.

Example 1:

Input: nums = [-2,0,1,3], target = 2
Output: 2
Explanation: Because there are two triplets which sums are less than 2:
[-2,0,1]
[-2,0,3]

Example 2:

Input: nums = [], target = 0
Output: 0

Example 3:

Input: nums = [0], target = 0
Output: 0

Constraints:

n == nums.length
0 <= n <= 3500
-100 <= nums[i] <= 100
-100 <= target <= 100
The input is generated such that the answer is less than or equal to 10⁹.