【leetcode】Find Peak Element

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.

Note:

Your solution should be in logarithmic complexity.

利用二分法才能达到logarithmic的复杂度

当num[mid]<num[mid+1]时，peek一定在右边

当num[mid]>num[mid+1]时，peek一定在左边

 1 class Solution {
 2 
 3 public:
 4 
 5     int findPeakElement(const vector<int> &num) {
 6 
 7        
 8 
 9         int n=num.size();
10 
11         int left=0;
12 
13         int right=n-1;
14 
15         int result;
16 
17         int mid;       
18 
19         while(left<right)
20 
21         {
22 
23             mid=(left+right)/2;
24 
25             if(mid+1>n-1)
26 
27             {
28 
29                 return mid;
30 
31             }           
32 
33             if(num[mid]<num[mid+1])
34 
35             {
36 
37                 left=mid+1;  
38 
39             }
40 
41             else if(num[mid]>num[mid+1])
42 
43             {
44 
45                 right=mid;
46 
47             }             
48 
49         }
50 
51  
52 
53         return left;       
54 
55     }
56 
57 };