LeetCode 162 :Find Peak Element
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2022-03-14 22:52:45
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问题描述
A peak element is an element that is greater than its neighbors.
Given an input array nums, where nums[i] ≠ nums[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 nums[-1] = nums[n] = -∞.
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: 1 or 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.
思路
二分
java实现
class Solution {
public int findPeakElement(int[] nums) {
if( nums==null || nums.length==0)
return 0;
int start=0;
int end=nums.length-1;
while (start+1<end){
int mid=start+(end-start)/2;
if(nums[mid]>nums[mid-1] && nums[mid]>nums[mid+1])//是峰值
return mid;
else if(nums[mid]<nums[mid-1] && nums[mid]<nums[mid+1])//是低谷值,左右两侧一点还有峰值,因为题目说是先上升后下降
start=mid;
else if(nums[mid]<nums[mid+1])//位于上升区间 mid以后半部分肯定有下降,存在峰值
start=mid;
else if(nums[mid]>nums[mid+1])//位于下降区间 mid以前半部分肯定有上升,存在峰值
end=mid;
}
if (nums[start]>nums[end])
return start;
else
return end;
}
}
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