1 Description #
source: https://leetcode.com/problems/find-peak-element/
A peak element is an element that is strictly greater than its neighbors.
Given an integer array nums, find a peak element, and return its index. If the array contains multiple peaks, return the index to any of the peaks.
You may imagine that nums[-1] = nums[n] = -∞.
You must write an algorithm that runs in O(log n) time.
Example 1:
Input: nums = [1,2,3,1]
Output: 2
Explanation: 3 is a peak element and your function should return the index number 2.
Example 2:
Input: nums = [1,2,1,3,5,6,4]
Output: 5
Explanation: Your function can return either index number 1 where the peak element is 2, or index number 5 where the peak element is 6.
Constraints:
1 <= nums.length <= 1000-2^31 <= nums[i] <= 2^31 - 1nums[i] != nums[i + 1]for all validi.
2 Solution #
class Solution {
public:
int findPeakElement(vector<int> &nums) {
// Time complexity: O(logN)
// Space complexity: O(1)
int low = 0;
int high = nums.size();
int mid = 0;
while (low <= high) {
mid = (low + high) / 2;
if (mid != 0 && nums[mid] < nums[mid - 1]) {
high = mid - 1;
} else if (mid != nums.size() - 1 && nums[mid] < nums[mid + 1]) {
low = mid + 1;
} else {
return mid;
}
}
return mid;
}
};