0976-Largest Perimeter Triangle.md

Largest Perimeter Triangle

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 is impossible to form any triangle of non-zero area, return 0.

Example 1:

Input: [2,1,2]
Output: 5

Example 2:

Input: [1,2,1]
Output: 0

Example 3:

Input: [3,2,3,4]
Output: 10

Example 4:

Input: [3,6,2,3]
Output: 8

Note:

1. 3 <= A.length <= 10000
2. 1 <= A[i] <= 10^6

Solution

O(A*log[base 2](A)+A) Time:

class Solution {
    func largestPerimeter(_ A: [Int]) -> Int {
        let A = A.sorted()
        for i in stride(from: A.count-3, through: 0, by: -1) {
            if A[i] + A[i+1] > A[i+2] {
                return A[i] + A[i+1] + A[i+2]
            }
        }
        return 0
    }
}