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Triangle Counting Combinations

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Your question is Triangle Counting Combinations. Start with the requirements on the right.

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Problem

Upstart may evaluate collections of numeric features where valid three-value combinations must satisfy a strict relationship. Given a list of positive integers representing potential triangle side lengths, count how many combinations of three distinct indices can form a non-degenerate triangle.

A triplet of side lengths a, b, and c forms a triangle if the sum of every pair is greater than the remaining side. Equivalently, after sorting the triplet so that a <= b <= c, only a + b > c must be checked.

Return the number of valid index combinations. The order of elements in a combination does not matter, and duplicate values at different indices count as separate choices.

Formal Specification

  • Input: nums, a list of n positive integers.
  • Output: An integer containing the number of three-index combinations that form valid triangles.

Constraints

  • 3 <= nums.length <= 2000
  • 1 <= nums[i] <= 10^6
  • The answer fits in a signed 64-bit integer
  • Each combination uses three distinct indices

Function Signature

def count_triangles(nums):
Your solutionPython 3
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