Leetcode--Find Peak Element

Find Peak Number:

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.

解題思路：

想法1：就是找到該數組的最大值，最大值肯定滿足該題要求。查找最大值可以用順序查找，時間複雜度爲O(n)。代碼如下：

<span style="font-family:Times New Roman;">class Solution {
public:
    int findPeakElement(const vector<int> &num) {
        for(int i=0; i<num.size(); i++)
        {  
		    if(num[i]  > num[i+1])  
		    	return i; 
        }
        return num.size()-1; 
    }
};</span>

想法2：查找最大值還可以使用折半查找方法。時間複雜度爲O(logn)

<span style="font-family:Times New Roman;">class Solution {
public:
    int findPeakElement(const vector<int> &num) {
       int left=0;
       int right = num.size()-1;
       while(left <= right)
       {
           if(left == right)
            return left;
            int mid = (left + right)/2;
            if(num[mid] < num[mid+1])
                left = mid+1;
            else
                right = mid;
       }
    }
};</span>