【LeetCode】162. Find Peak Element (3 solutions)

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.

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.

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Note:
Your solution should be in logarithmic complexity.

这题就是求序列最大值。顺序查找或二分查找均可。

满足复杂度要求的话需要用二分查找。

解法一：顺序查找

class Solution {
public:
int findPeakElement(vector<int>& nums) {
int n = nums.size();
if(n == 1)
return 0;
if(nums[0] > nums[1])
return 0;
if(nums[n-1] > nums[n-2])
return n-1;
for(int i = 1; i < n-1; i ++)
if(nums[i] > nums[i-1] && nums[i] > nums[i+1])
return i;
}
};

解法二：二分查找（递归）

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

解法三：二分查找（迭代）

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