162. Find Peak Element

1. Question

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.

2. Examples

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.

3. Constraints

• 1 <= nums.length <= 1000
• -2³¹ <= nums[i] <= 2³¹ - 1
• nums[i] != nums[i + 1] for all valid i.

4. References

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/find-peak-element 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

5. Solutions

nums[-1] = nums[n] = -∞

二分情况下，a>b峰值一定在a侧出现。

class Solution {
    public int findPeakElement(int[] nums) {
        int left = 0;
        int right = nums.length - 1;
        while (left < right) {
            int mid = (left + right) / 2;
            if (nums[mid] < nums[mid + 1]) {
                left = mid + 1;
            } else {
                right = mid;
            }
        }
        return left;
    }
}