Question

A peak element is an element that is greater than its neighbors.

Given an input array where num[i] ≠ num[i+1], find a peak element and return its index.

The array may contain multiple peaks, in that case return the index to any one of the peaks is fine.

You may imagine that num[-1] = num[n] = -∞.

For example, in array [1, 2, 3, 1], 3 is a peak element and your function should return the index number 2.

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Note:
Your solution should be in logarithmic complexity.

Quick Hints

• Binary search

Solution

• A very good guide to solve this problem is present here
• The idea here is that we can use binary search. If nums(i) < nums(i+1) we know that the peak exists towards the right and we can make our low boundary mid + 1 else we make our high boundary mid since the mid itself can be peak.

Time complexity

O (log n)

Space complexity

O(1) except the space on stack

Notes