Is Valid Triangle Number easy or hard?

Valid Triangle Number is generally considered a medium difficulty problem. The triangle inequality is simple, but recognizing that sorting enables a two-pointer counting strategy is the key insight.

Valid Triangle Number Python/Java solution

Most implementations follow the same pattern: sort the array, iterate the largest side index from the end, and run a two-pointer search for valid pairs. The logic translates directly to Python, Java, C++, C#, and JavaScript with identical O(n^2) complexity.

How to solve Valid Triangle Number in O(n^2)?

First sort the array so the largest side is easy to fix. For each index k as the largest side, place two pointers at the start and just before k. If nums[left] + nums[right] > nums[k], then all elements between left and right form valid triangles with right, so add right - left and move the right pointer.

What is the best approach for Valid Triangle Number?

The best approach is sorting the array followed by a two-pointer scan. After sorting, fix the largest side and use two pointers to find pairs whose sum is greater than that side. This counts multiple valid triangles at once and runs in O(n^2) time with O(1) extra space.

Is Valid Triangle Number asked at Google/Amazon/Meta?

Triangle inequality and two-pointer counting problems frequently appear in interviews at companies like Amazon, Meta, and Google. Variations of this problem test understanding of sorting, pointer movement, and counting combinations efficiently.

What data structure is used in Valid Triangle Number?

The core data structure is a simple array. The algorithm relies on sorting the array and then applying the two-pointer technique to scan combinations efficiently without extra memory.

What is the time complexity of Valid Triangle Number?

The optimal solution runs in O(n^2) time after sorting the array. Sorting takes O(n log n), and the two-pointer scan processes each element pair at most once. The brute force approach is O(n^3) because it checks every triplet.

Description Solution Video 10:59

611. Valid Triangle Number

Medium57.4% acceptance9 companies

Given an integer array nums, return the number of triplets chosen from the array that can make triangles if we take them as side lengths of a triangle.

Example 1:

Input: nums = [2,2,3,4]
Output: 3
Explanation: Valid combinations are: 
2,3,4 (using the first 2)
2,3,4 (using the second 2)
2,2,3

Example 2:

Input: nums = [4,2,3,4]
Output: 4

Constraints:

1 <= nums.length <= 1000
0 <= nums[i] <= 1000

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611. Valid Triangle Number

Asked at 9 companies

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