Description
https://leetcode.com/problems/peak-index-in-a-mountain-array/
Let’s call an array A
a mountain if the following properties hold:
A.length >= 3
- There exists some
0 < i < A.length - 1
such thatA[0] < A[1] < ... A[i-1] < A[i] > A[i+1] > ... > A[A.length - 1]
Given an array that is definitely a mountain, return any i
such that A[0] < A[1] < ... A[i-1] < A[i] > A[i+1] > ... > A[A.length - 1]
.
Example 1:
Input: [0,1,0] Output: 1
Example 2:
Input: [0,2,1,0] Output: 1
Note:
3 <= A.length <= 10000
0 <= A[i] <= 10^6
- A is a mountain, as defined above.
Explanation
binary search
Python Solution
class Solution:
def peakIndexInMountainArray(self, A: List[int]) -> int:
if not A:
return -1
start = 0
end = len(A) - 1
while start + 1 < end:
mid = start + (end - start) // 2
if A[mid] > A[mid - 1] and A[mid] > A[mid + 1]:
return mid
elif A[mid] < A[mid + 1]:
start = mid
else:
end = mid
if A[end] > A[start]:
return end
else:
return start
return -1
- Time complexity: ~log(N)
- Space complexity: ~1