Leetcode – Largest Perimeter Triangle Solution in Python

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Triangular problems, with their inherent geometrical and mathematical properties, offer intriguing challenges in the realm of algorithmic programming. The “Largest Perimeter Triangle” problem on Leetcode is one such challenge that requires a mix of mathematical understanding and algorithmic flair. In this comprehensive article, we’ll decode the problem, look at its potential solutions, and delve into a Python-based implementation.

Problem Statement

Given an array A of positive lengths, return the largest perimeter of a triangle with non-zero area, formed from 3 of these lengths. If it’s impossible to form any triangle of non-zero area, return 0.

Understanding the Problem

To form a valid triangle, the sum of its two shorter sides must be greater than the length of its longest side. Given this condition, the task becomes finding the three largest numbers in the array that can satisfy this triangular inequality.

Key Insight

Given that we want the largest perimeter, we should aim to use the largest numbers possible. Thus, if we sort the array in descending order and traverse it, looking for the first triplet that satisfies the triangular inequality, that triplet will give the triangle with the maximum perimeter.

Algorithm to Solve the Problem

  1. Sort the given array in descending order.
  2. Traverse the sorted array and for every triplet (considering three consecutive elements), check if they can form a triangle:
    • If they can, return their sum (which will be the largest perimeter).
    • Otherwise, continue to the next triplet.
  3. If no valid triangle can be formed, return 0.

Python Code Solution

Let’s translate our algorithm into a Python function:

def largestPerimeter(A):
    for i in range(len(A) - 2):
        if A[i] < A[i + 1] + A[i + 2]:
            return A[i] + A[i + 1] + A[i + 2]
    return 0

Complexity Analysis

  • Time Complexity: O(N log N) – The most time-consuming operation here is the sorting of the array.
  • Space Complexity: O(1) – We didn’t use any additional data structures that scale with the size of the input.

Testing the Solution

It’s essential to test the solution to validate its correctness:

print(largestPerimeter([2,1,2]))       # Expected output: 5 (Triangle sides: 2, 2, 1)
print(largestPerimeter([1,2,1]))       # Expected output: 0 (No valid triangle can be formed)
print(largestPerimeter([3,2,3,4]))     # Expected output: 10 (Triangle sides: 4, 3, 3)
print(largestPerimeter([3,6,2,3]))     # Expected output: 8 (Triangle sides: 3, 3, 2)


The “Largest Perimeter Triangle” problem underscores the significance of understanding the inherent properties of the problem domain (in this case, the properties of triangles) to arrive at an optimized solution. Problems like this emphasize not just coding skills, but also the importance of mathematical comprehension in the realm of computer programming.

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