PeakIndexInAMountainArray.js

// Source : https://leetcode.com/problems/peak-index-in-a-mountain-array
// Author : Dean Shi
// Date   : 2018-08-13

/***************************************************************************************
 * Let's call an array A a mountain if the following properties hold:
 *
 * A.length >= 3
 * There exists some 0 < i < A.length - 1 such that A[0] < A[1] < ... A[i-1] < A[i] >
 * A[i+1] > ... > A[A.length - 1]
 *
 * Given an array that is definitely a mountain, return any i such that A[0] < A[1] < ..
 * . A[i-1] < A[i] > A[i+1] > ... > A[A.length - 1].
 *
 * Example 1:
 *
 * Input: [0,1,0]
 * Output: 1
 *
 * Example 2:
 *
 * Input: [0,2,1,0]
 * Output: 1
 *
 * Note:
 *
 * 3 <= A.length <= 10000
 * 0 <= A[i] <= 10^6
 * A is a mountain, as defined above.
 *
 ***************************************************************************************/

/**
 * @param {number[]} A
 * @return {number}
 */
var peakIndexInMountainArray = function(A) {
    let i = 0
    while (A[i] < A[i+1]) i++
    return i
};