PeakElement.cpp

// Find a peak element's index in an array

#include<bits/stdc++.h>
using namespace std;

int getPeakElement(vector<int> &nums) {

    int n = nums.size();

    if (n == 1)
        return 0;

    int low = 0, high = n - 1;

    while (low <= high) {

        int mid = low + (high - low) / 2;

        if ((mid > 0) && (mid < n -1)) {

            if ((nums[mid] >= nums[mid + 1]) && (nums[mid] >= nums[mid - 1]))
                return mid;
            else if (nums[mid - 1] > nums[mid])
                high = mid - 1;
            else
                low = mid + 1;
        }
        else if (mid == 0) {

            if (nums[mid] >= nums[mid + 1])
                return 0;
            else
                return 1;
        }
        else if (mid == n - 1) {

            if (nums[n - 1] >= nums[n - 2])
                return n - 1;
            else
                return n - 2;
        }
    }
}

int main() {

    int n;
    cout << "Enter the number of elements: ";
    cin >> n;

    vector<int> nums;

    cout << "Enter " << n << " elements: ";
    for (int i = 0; i < n; ++i) {
        int x;
        cin >> x;
        nums.emplace_back(x);
    }

    int res = getPeakElement(nums);
    cout << "Peak element index: " << res << '\n';

    return 0;
}