mountainArray2.js

/**
 * @param {number[]} A
 * @return {boolean}
 */
var validMountainArray = function(A) {
    if (A.length < 3) {
        return false;
    }
    let i = 0;
    let j = 1;
    while (A[i] < A[j] && i < A.length) {
        i++;
        j++;
    }
    // i should be at the peak
    
    if (i == 0 || j >= A.length) {
        return false;
    }
    while (A[i] > A[j] && j < A.length) {
        i++;
        j++;
    }
    
    if (j < A.length) {
        return false;
    } else {
        return true;
    }
    
};