Simple mountain
Input
arr = [1,2,3,4,3,2,1]
Output
3
The peak value 4 is at index 3.
Full lesson preview
Find a peak in a mountain array using binary search
Locate the peak index in a mountain (bitonic) array in O(log n) time.
Python practice16 minSearching & SortingIntermediateLast updated March 25, 2026
Problem statement
A mountain array (also called bitonic) is an array where elements strictly increase to a single peak, then strictly decrease. Given such an array, return the index of the peak element. Use an algorithm with O(log n) time complexity.
Task
Apply binary search variant to find the maximum (peak) in an array that strictly increases then strictly decreases.
Examples
Input format
One argument: arr (list of integers) with length >= 3 and a valid mountain shape.
Output format
An integer index of the peak element.
Constraints
Time complexity: O(log n). Array length n >= 3. The array strictly increases then strictly decreases with exactly one peak.
Samples
Sample 1
Input
arr = [0,2,1,0]
Output
1
2 is the peak at index 1.