852

852. Peak Index in a Mountain Array (Medium)

An array arr a mountain if the following properties hold:

• arr.length >= 3
• There exists some i with 0 < i < arr.length - 1 such that:
  • arr[0] < arr[1] < ... < arr[i - 1] < arr[i]
  • arr[i] > arr[i + 1] > ... > arr[arr.length - 1]

Given a mountain array arr, return the index i such that arr[0] < arr[1] < ... < arr[i - 1] < arr[i] > arr[i + 1] > ... > arr[arr.length - 1].

You must solve it in O(log(arr.length)) time complexity.

 

Example 1:

Input: arr = [0,1,0]
Output: 1

Example 2:

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

Example 3:

Input: arr = [0,10,5,2]
Output: 1

 

Constraints:

• 3 <= arr.length <= 105
• 0 <= arr[i] <= 106
• arr is guaranteed to be a mountain array.

Companies: Amazon, Adobe, Bloomberg

Related Topics:
Array, Binary Search

Similar Questions:

• Find Peak Element (Medium)
• Find in Mountain Array (Hard)
• Minimum Number of Removals to Make Mountain Array (Hard)

Solution 1. Binary Search

// OJ: https://leetcode.com/problems/peak-index-in-a-mountain-array/
// Author: github.com/lzl124631x
// Time: O(logN)
// Space: O(1)
class Solution {
public:
    int peakIndexInMountainArray(vector<int>& A) {
        int L = 1, R = A.size() - 2;
        while (L <= R) {
            int M = (L + R) / 2;
            if (A[M] > A[M - 1]) L = M + 1;
            else R = M - 1;
        }
        return R;
    }
};