找到一个峰值元素

给定一个整数数组。在其中找到一个峰值元素。如果一个数组元素不小于它的邻居，那么它就是一个峰值。对于角元素，我们只需要考虑一个邻居。

例子：

Input: array[]= {5, 10, 20, 15}
Output: 20
The element 20 has neighbours 10 and 15,
both of them are less than 20.

Input: array[] = {10, 20, 15, 2, 23, 90, 67}
Output: 20 or 90
The element 20 has neighbours 10 and 15, 
both of them are less than 20, similarly 90 has neighbours 23 and 67.

以下角落案例可以更好地了解问题。

1. 如果输入数组按严格递增顺序排序，则最后一个元素始终是峰值元素。例如，50 是 {10, 20, 30, 40, 50} 中的峰元素。
2. 如果输入数组按严格降序排序，则第一个元素始终是峰值元素。 100 是 {100, 80, 60, 50, 20} 中的峰值元素。
3. 如果输入数组的所有元素都相同，则每个元素都是峰值元素。

从上面的例子可以清楚地看出，输入数组中总是有一个peak元素。

朴素的方法：可以遍历数组并返回其邻居小于该元素的元素。
算法：

1. 如果在数组中，第一个元素大于第二个或最后一个元素大于倒数第二个，则打印相应的元素并终止程序。
2. else 从第二个索引到倒数第二个索引遍历数组
3. 如果对于元素array[i] ，它大于它的两个邻居，即array[i] > array[i-1] 和 array[i] > array[i+1] ，然后打印该元素并终止。

C++

// A C++ program to find a peak element
#include 
using namespace std;
 
// Find the peak element in the array
int findPeak(int arr[], int n)
{
    // first or last element is peak element
    if (n == 1)
      return 0;
    if (arr[0] >= arr[1])
        return 0;
    if (arr[n - 1] >= arr[n - 2])
        return n - 1;
 
    // check for every other element
    for (int i = 1; i < n - 1; i++) {
 
        // check if the neighbors are smaller
        if (arr[i] >= arr[i - 1] && arr[i] >= arr[i + 1])
            return i;
    }
}
 
// Driver Code
int main()
{
    int arr[] = { 1, 3, 20, 4, 1, 0 };
    int n = sizeof(arr) / sizeof(arr[0]);
    cout << "Index of a peak point is "
         << findPeak(arr, n);
    return 0;
}
 
// This code is contributed by Arnab Kundu

Java

// A Java program to find a peak element
import java.util.*;
 
class GFG{
 
// Find the peak element in the array
static int findPeak(int arr[], int n)
{
     
    // First or last element is peak element
    if (n == 1)
      return 0;
    if (arr[0] >= arr[1])
        return 0;
    if (arr[n - 1] >= arr[n - 2])
        return n - 1;
 
    // Check for every other element
    for(int i = 1; i < n - 1; i++)
    {
         
        // Check if the neighbors are smaller
        if (arr[i] >= arr[i - 1] &&
            arr[i] >= arr[i + 1])
            return i;
    }
    return 0;
}
 
// Driver Code
public static void main(String[] args)
{
    int arr[] = { 1, 3, 20, 4, 1, 0 };
    int n = arr.length;
     
    System.out.print("Index of a peak point is " +
                     findPeak(arr, n));
}
}
 
// This code is contributed by Princi Singh

Python3

# A Python3 program to find a peak element
 
# Find the peak element in the array
def findPeak(arr, n) :
 
    # first or last element is peak element
    if (n == 1) :
      return 0
    if (arr[0] >= arr[1]) :
        return 0
    if (arr[n - 1] >= arr[n - 2]) :
        return n - 1
  
    # check for every other element
    for i in range(1, n - 1) :
  
        # check if the neighbors are smaller
        if (arr[i] >= arr[i - 1] and arr[i] >= arr[i + 1]) :
            return i
             
# Driver code.
arr = [ 1, 3, 20, 4, 1, 0 ]
n = len(arr)
print("Index of a peak point is", findPeak(arr, n))
 
# This code is contributed by divyeshrabadiya07

C#

// A C# program to find a peak element
using System;
 
public class GFG{
 
// Find the peak element in the array
static int findPeak(int []arr, int n)
{
     
    // First or last element is peak element
    if (n == 1)
      return 0;
    if (arr[0] >= arr[1])
        return 0;
    if (arr[n - 1] >= arr[n - 2])
        return n - 1;
 
    // Check for every other element
    for(int i = 1; i < n - 1; i++)
    {
         
        // Check if the neighbors are smaller
        if (arr[i] >= arr[i - 1] &&
            arr[i] >= arr[i + 1])
            return i;
    }
    return 0;
}
 
// Driver Code
public static void Main(String[] args)
{
    int []arr = { 1, 3, 20, 4, 1, 0 };
    int n = arr.Length;
     
    Console.Write("Index of a peak point is " +
                     findPeak(arr, n));
}
}
 
  
 
// This code is contributed by 29AjayKumar

Javascript

      // A JavaScript program to find a peak element
 
      // Find the peak element in the array
      function findPeak(arr, n)
      {
       
        // first or last element is peak element
        if (n == 1) return 0;
        if (arr[0] >= arr[1]) return 0;
        if (arr[n - 1] >= arr[n - 2]) return n - 1;
 
        // check for every other element
        for (var i = 1; i < n - 1; i++)
        {
         
          // check if the neighbors are smaller
          if (arr[i] >= arr[i - 1] && arr[i] >= arr[i + 1]) return i;
        }
      }
 
      // Driver Code
      var arr = [1, 3, 20, 4, 1, 0];
      var n = arr.length;
      document.write("Index of a peak point is " + findPeak(arr, n));
       
      // This code is contributed by rdtank.
    

输出：

Index of a peak point is 2

复杂度分析：

• 时间复杂度： O(n)。
  需要一次遍历，所以时间复杂度为 O(n)
• 空间复杂度： O(1)。
  不需要额外的空间，所以空间复杂度是恒定的

有效的方法：分而治之，可用于在 O(Logn) 时间内找到峰值。这个想法是基于二分搜索技术来检查中间元素是否是峰值元素。如果中间元素不是峰值元素，则检查右侧元素是否大于中间元素，则右侧始终存在峰值元素。如果左侧的元素大于中间的元素，则左侧总是有一个峰值元素。形成递归，可以在 log n 时间内找到峰值元素。

算法：

1. 创建两个变量l和r ，初始化l = 0和r = n-1
2. 迭代下面的步骤直到l <= r ，下限小于上限
3. 检查中间值或索引mid = (l+r)/2是否是峰值元素，如果是，则打印元素并终止。
4. 否则，如果中间元素左侧的元素更大，则检查左侧的峰值元素，即更新r = mid – 1
5. 否则，如果中间元素右侧的元素更大，则检查右侧的峰值元素，即更新l = mid + 1

C++

// A C++ program to find a peak element
// using divide and conquer
#include 
using namespace std;
 
// A binary search based function
// that returns index of a peak element
int findPeakUtil(int arr[], int low,
                 int high, int n)
{
    // Find index of middle element
    // (low + high)/2
    int mid = low + (high - low) / 2;
 
    // Compare middle element with its
    // neighbours (if neighbours exist)
    if ((mid == 0 || arr[mid - 1] <= arr[mid]) &&
        (mid == n - 1 || arr[mid + 1] <= arr[mid]))
        return mid;
 
    // If middle element is not peak and its
    // left neighbour is greater than it,
    // then left half must have a peak element
    else if (mid > 0 && arr[mid - 1] > arr[mid])
        return findPeakUtil(arr, low, (mid - 1), n);
 
    // If middle element is not peak and its
    // right neighbour is greater than it,
    // then right half must have a peak element
    else
        return findPeakUtil(
            arr, (mid + 1), high, n);
}
 
// A wrapper over recursive function findPeakUtil()
int findPeak(int arr[], int n)
{
    return findPeakUtil(arr, 0, n - 1, n);
}
 
// Driver Code
int main()
{
    int arr[] = { 1, 3, 20, 4, 1, 0 };
    int n = sizeof(arr) / sizeof(arr[0]);
    cout << "Index of a peak point is "
         << findPeak(arr, n);
    return 0;
}
 
// This code is contributed by rathbhupendra

C

// C program to find a peak
// element using divide and conquer
#include 
 
// A binary search based function that
// returns index of a peak element
int findPeakUtil(
    int arr[], int low, int high, int n)
{
    // Find index of middle element
    // (low + high)/2
    int mid = low + (high - low) / 2;
 
    // Compare middle element with
    // its neighbours (if neighbours exist)
    if ((mid == 0 || arr[mid - 1] <= arr[mid]) && (mid == n - 1 || arr[mid + 1] <= arr[mid]))
        return mid;
 
    // If middle element is not peak and
    // its left neighbour is greater
    // than it, then left half must have a peak element
    else if (mid > 0 && arr[mid - 1] > arr[mid])
        return findPeakUtil(arr, low, (mid - 1), n);
 
    // If middle element is not peak and
    // its right neighbour is greater
    // than it, then right half must have a peak element
    else
        return findPeakUtil(arr, (mid + 1), high, n);
}
 
// A wrapper over recursive function findPeakUtil()
int findPeak(int arr[], int n)
{
    return findPeakUtil(arr, 0, n - 1, n);
}
 
/* Driver program to check above functions */
int main()
{
    int arr[] = { 1, 3, 20, 4, 1, 0 };
    int n = sizeof(arr) / sizeof(arr[0]);
    printf(
        "Index of a peak point is %d", findPeak(arr, n));
    return 0;
}

Java

// A Java program to find a peak
// element using divide and conquer
import java.util.*;
import java.lang.*;
import java.io.*;
 
class PeakElement {
    // A binary search based function
    // that returns index of a peak element
    static int findPeakUtil(
        int arr[], int low, int high, int n)
    {
        // Find index of middle element
        // (low + high)/2
        int mid = low + (high - low) / 2;
 
        // Compare middle element with its
        // neighbours (if neighbours exist)
        if ((mid == 0 || arr[mid - 1] <= arr[mid])
            && (mid == n - 1 || arr[mid + 1] <= arr[mid]))
            return mid;
 
        // If middle element is not peak
        // and its left neighbor is
        // greater than it, then left half
        // must have a peak element
        else if (mid > 0 && arr[mid - 1] > arr[mid])
            return findPeakUtil(arr, low, (mid - 1), n);
 
        // If middle element is not peak
        // and its right neighbor
        // is greater than it, then right
        // half must have a peak
        // element
        else
            return findPeakUtil(
                arr, (mid + 1), high, n);
    }
 
    // A wrapper over recursive function
    // findPeakUtil()
    static int findPeak(int arr[], int n)
    {
        return findPeakUtil(arr, 0, n - 1, n);
    }
 
    // Driver method
    public static void main(String[] args)
    {
        int arr[] = { 1, 3, 20, 4, 1, 0 };
        int n = arr.length;
        System.out.println(
            "Index of a peak point is " + findPeak(arr, n));
    }
}

蟒蛇3

# A python3 program to find a peak
# element element using divide and conquer
 
# A binary search based function
# that returns index of a peak element
def findPeakUtil(arr, low, high, n):
     
     # Find index of middle element
     # (low + high)/2
     mid = low + (high - low)/2
     mid = int(mid)
     
    # Compare middle element with its
    # neighbours (if neighbours exist)
    if ((mid == 0 or arr[mid - 1] <= arr[mid]) and
        (mid == n - 1 or arr[mid + 1] <= arr[mid])):
        return mid
 
 
    # If middle element is not peak and
    # its left neighbour is greater
    # than it, then left half must
    # have a peak element
    elif (mid > 0 and arr[mid - 1] > arr[mid]):
        return findPeakUtil(arr, low, (mid - 1), n)
 
    # If middle element is not peak and
    # its right neighbour is greater
    # than it, then right half must
    # have a peak element
    else:
        return findPeakUtil(arr, (mid + 1), high, n)
 
 
# A wrapper over recursive
# function findPeakUtil()
def findPeak(arr, n):
 
    return findPeakUtil(arr, 0, n - 1, n)
 
 
# Driver code
arr = [1, 3, 20, 4, 1, 0]
n = len(arr)
print("Index of a peak point is", findPeak(arr, n))
     
# This code is contributed by
# Smitha Dinesh Semwal

C＃

// A C# program to find
// a peak element element
// using divide and conquer
using System;
 
class GFG {
 
    // A binary search based
    // function that returns
    // index of a peak element
    static int findPeakUtil(int[] arr, int low,
                            int high, int n)
    {
        // Find index of
        // middle element
        int mid = low + (high - low) / 2;
 
        // Compare middle element with
        // its neighbours (if neighbours
        // exist)
        if ((mid == 0 || arr[mid - 1] <= arr[mid]) && (mid == n - 1 || arr[mid + 1] <= arr[mid]))
            return mid;
 
        // If middle element is not
        // peak and its left neighbor
        // is greater than it, then
        // left half must have a
        // peak element
        else if (mid > 0 && arr[mid - 1] > arr[mid])
            return findPeakUtil(arr, low,
                                (mid - 1), n);
 
        // If middle element is not
        // peak and its right neighbor
        // is greater than it, then
        // right half must have a peak
        // element
        else
            return findPeakUtil(arr, (mid + 1),
                                high, n);
    }
 
    // A wrapper over recursive
    // function findPeakUtil()
    static int findPeak(int[] arr,
                        int n)
    {
        return findPeakUtil(arr, 0,
                            n - 1, n);
    }
 
    // Driver Code
    static public void Main()
    {
        int[] arr = { 1, 3, 20,
                      4, 1, 0 };
        int n = arr.Length;
        Console.WriteLine("Index of a peak "
                          + "point is " + findPeak(arr, n));
    }
}
 
// This code is contributed by ajit

PHP

 0 &&
             $arr[$mid - 1] > $arr[$mid])
        return findPeakUtil($arr, $low,
                           ($mid - 1), $n);
 
    // If middle element is not peak
    // and its right neighbour is
    // greater than it, then right
    // half must have a peak element
    else return(findPeakUtil($arr, ($mid + 1),
                             $high, $n));
}
 
// A wrapper over recursive
// function findPeakUtil()
function findPeak($arr, $n)
{
    return floor(findPeakUtil($arr, 0,
                              $n - 1, $n));
}
 
// Driver Code
$arr = array(1, 3, 20, 4, 1, 0);
$n = sizeof($arr);
echo "Index of a peak point is ",
              findPeak($arr, $n);
 
// This code is contributed by ajit
?>

Javascript

 
// A Javascript program to find a peak element
// using divide and conquer
 
// A binary search based function
// that returns index of a peak element
function findPeakUtil(arr, low, high, n)
{
    // Find index of middle element
    // (low + high)/2
    var mid = low + parseInt((high - low) / 2);
 
    // Compare middle element with its
    // neighbours (if neighbours exist)
    if ((mid == 0 || arr[mid - 1] <= arr[mid]) &&
        (mid == n - 1 || arr[mid + 1] <= arr[mid]))
        return mid;
 
    // If middle element is not peak and its
    // left neighbour is greater than it,
    // then left half must have a peak element
    else if (mid > 0 && arr[mid - 1] > arr[mid])
        return findPeakUtil(arr, low, (mid - 1), n);
 
    // If middle element is not peak and its
    // right neighbour is greater than it,
    // then right half must have a peak element
    else
        return findPeakUtil(
            arr, (mid + 1), high, n);
}
 
// A wrapper over recursive function findPeakUtil()
 
function findPeak(arr, n)
{
    return findPeakUtil(arr, 0, n - 1, n);
}
 
// Driver Code
var arr = [ 1, 3, 20, 4, 1, 0 ];
var n = arr.length;
document.write("Index of a peak point is "
    + findPeak(arr, n));
 
 

输出：

Index of a peak point is 2

复杂度分析：

• 时间复杂度： O(Logn)。
  其中 n 是输入数组中的元素数。在每一步中，我们的搜索都变成了一半。所以可以比作二分查找，所以时间复杂度是O(log n)
• 空间复杂度： O(1)。
  不需要额外的空间，所以空间复杂度是恒定的。

锻炼：
考虑以下修改后的峰值元素定义。如果数组元素大于其相邻元素，则该数组元素是一个峰值。请注意，数组可能不包含具有此修改定义的峰值元素。

参考：
http://courses.csail.mit.edu/6.006/spring11/lectures/lec02.pdf
http://www.youtube.com/watch?v=HtSuA80QTyo

询问：亚马逊

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