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611. Valid Triangle Number

Difficulty Topics

Description

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

valid-triangle-number.py
class Solution:
    def triangleNumber(self, nums: List[int]) -> int:
        nums.sort()
        n = len(nums)
        res = 0

        for k in range(n - 1, 1, -1):
            i, j = 0, k - 1
            while i < j:
                if nums[i] + nums[j] > nums[k]:
                    res += j - i
                    j -= 1
                else:
                    i += 1

        return res