You are given an integer array nums of length n. An array is trionic if there exist indices 0 < p < q < n − 1 such that: nums[0...p] is strictly increasing, nums[p...q] is strictly decreasing, nums[q...n − 1] is strictly increasing. Return true if

isTrionic.js

/**
 * @param {number[]} nums
 * @return {boolean}
 */
var isTrionic = function(nums) {
    const n = nums.length;

    // Need at least 4 elements to form inc → dec → inc
    if (n < 4) return false;

    let i = 0;

    // -------------------------------
    // Phase 1: strictly increasing
    // -------------------------------
    while (i + 1 < n && nums[i] < nums[i + 1]) {
        i++;
    }

    // If we never increased or we stopped at the last element,
    // we cannot form the required pattern.
    if (i === 0 || i === n - 1) return false;

    // -------------------------------
    // Phase 2: strictly decreasing
    // -------------------------------
    let peak = i; // p index
    while (i + 1 < n && nums[i] > nums[i + 1]) {
        i++;
    }

    // If we never decreased or we stopped at the last element,
    // we cannot form the required pattern.
    if (i === peak || i === n - 1) return false;

    // -------------------------------
    // Phase 3: strictly increasing again
    // -------------------------------
    let valley = i; // q index
    while (i + 1 < n && nums[i] < nums[i + 1]) {
        i++;
    }

    // If we never increased in this final phase, it's invalid.
    if (i === valley) return false;

    // If we consumed the entire array successfully, it's trionic.
    return i === n - 1;
};