852. Peak Index in a Mountain Array

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852. Peak Index in a Mountain Array

Description

Difficulty: Medium

Related Topics: Array, Binary Search

An array arr is 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:

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

Example 2:

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

Example 3:

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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.

Hints/Notes

  • Variant binary search

Solution

Language: C++

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class Solution {
public:
    int peakIndexInMountainArray(vector<int>& arr) {
        int left = 0, right = arr.size() - 1;
        while (left < right) {
            int mid = left + (right - left) / 2;
            if (arr[mid] > arr[mid + 1]) {
                right = mid;
            } else {
                left = mid + 1;
            }
        }
        return left;
    }
};