611. Valid Triangle Number

Problem Link

Description

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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

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Python3

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