dsa-competitive-programming/leetcode/lc3637/trionicArray.cpp

class Solution {
    public : 
        bool isTrionic(vector<int> &nums){
            int n =nums.size();
            if(n<3) return false;
            int p = 0;
            int q = n-1;
            while(p+1 <n && nums[p] < nums[p+1]) p++;
            while(q-1>=0 && nums[q] > nums[q-1]) q--;
            if(p==0 || q==n-1) return false;
            while(p+1<=q){
                if(nums[p] > nums[p+1]) p++;
                else return false;
            }
            return p==q;
        }
};

// Uses two pointers, TC : o(n) , SC : o(1)
// finds valid p and q if they exist, adn checks for strictly decreasing between p...q