[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
= -∞

.

For example, in array

[1, 2, 3, 1]

, 3 is a peak element and your function should return
the index number 2.

解法1

class Solution {
public:
int findPeakElement(const vector<int> &num) {
int len = num.size() - 1;
int low = 0,high = len;
if(high == 1) return num[0] > num[1] ? 0 : 1;
while(low <= high){
int mid = (low + high)/2;
if(low == high) return mid;
if(mid == 0) return num[0] > num[1] ? 0 : 1;
if(mid == len) return num[len - 1] > num[len] ? len - 1 : len;
if(num[mid] > num[mid - 1] && num[mid] > num[mid + 1])
return mid;
else if(num[mid] < num[mid - 1])
high = mid - 1;
else if(num[mid] < num[mid + 1])
low = mid + 1;
}
}
};

解法2

class Solution {
public:
int findPeakElement(const vector<int> &num)
{
int low = 0;
int high = num.size()-1;

while(low < high)
{
int mid1 = (low+high)/2;
int mid2 = mid1+1;
if(num[mid1] < num[mid2])
low = mid2;
else
high = mid1;
}
return low;
}
};