[LeetCode] Find Peak Element

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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Credits:

Special thanks to ​​@ts​​ for adding this problem and creating all test cases.

 

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分析：

 

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class Solution {
    public:
        int findPeakElement(vector<int>& nums)
        {   
            if(nums.size() == 0)
                return -1; 
            if(nums.size() == 1)
                return 0;
            if(nums[0] > nums[1])
                return 0;

            int size = nums.size();
            if(nums[size - 1] > nums[size - 2]) 
                return size - 1;

            int low = 1;
            int high = size - 2;
            int mid;

            while(low < high)
            {   
                mid = (low + high)/2;
                if(nums[mid] < nums[mid+1])
                {   
                    low = mid+1;
                }   
                else if(nums[mid] < nums[mid-1])
                {
                    high = mid-1;
                }
                else
                    return mid;
            }
            return low;


        }
};