Valid Triangle Number

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
SOLUTION:
class Solution:
    def triangleNumber(self, nums: List[int]) -> int:
        nums.sort()
        n = len(nums)
        ctr = 0
        i = 2
        while i < n:
            a = 0
            b = i - 1
            while a < b:
                if nums[a] + nums[b] > nums[i]:
                    ctr += (b - a)
                    b -= 1
                else:
                    a += 1
            i += 1
        return ctr

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