Peak Index in a Mountain Array Problem & Solution
Let's call an array arr a mountain if the following properties hold:
arr.length >= 3- There exists some
iwith0 < i < arr.length - 1such that:arr[0] < arr[1] < ... arr[i-1] < arr[i]arr[i] > arr[i+1] > ... > arr[arr.length - 1]
Given an integer array arr that is guaranteed to be a mountain, return any i such that arr[0] < arr[1] < ... arr[i - 1] < arr[i] > arr[i + 1] > ... > arr[arr.length - 1].
See the peak index in a mountain array problem on LeetCode.
C++ Solution
#pragma GCC optimize("Ofast")
#pragma GCC optimization("max-inline-insns-recursive-auto")
static const int _=[](){std::ios::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);return 0;}();
class Solution {
public:
int peakIndexInMountainArray(vector<int>& arr) {
return bs(arr, 0, arr.size() - 1);
}
private:
int bs(vector<int> &arr, int low, int high) {
int mid = low + (high - low) / 2;
if (arr[mid] > arr[mid - 1] &&
arr[mid] > arr[mid + 1]) {
return mid;
}
if (arr[mid] > arr[mid - 1]) {
return bs(arr, mid + 1, high);
} else {
return bs(arr, low, high - 1);
}
}
};