# Minimum peak elements from an array by their repeated removal at every iteration of the array

• Difficulty Level : Hard
• Last Updated : 19 May, 2021

Given an array arr[] consisting of N distinct positive integers, the task is to repeatedly find the minimum peak element from the given array and remove that element until all the array elements are removed.

Peak Element: Any element in the array is know as the peak element based on the following conditions:

• If arr[i – 1] < arr[i] > arr[i + 1], where 1 < i < N – 1, then arr[i] is the peak element.
• If arr[0] > arr[1], then arr[0] is the peak element, where N is the size of the array.
• If arr[N – 2] < arr[N – 1], then arr[N – 1] is the peak element, where N is the size of the array.

If more than one peak element exists in the array, then the minimum value among them needs to be printed.

Examples:

Input: arr[] = {1, 9, 7, 8, 2, 6}
Output: [6, 8, 9, 7, 2, 1]
Explanation:
First min peak = 6, as 2 < 6.
The array after removing min peak will be [1, 9, 7, 8, 2].
Second min peak = 8, as 7 < 8 > 2.
The array after removing min peak will be [1, 9, 7, 2]
Third min peak = 9, as 1 < 9 > 7.
The array after removing min peak will be [1, 7, 2]
Fourth min peak = 7, as 1 < 7 > 2.
The array after removing min peak will be  [1, 2]
Fifth min peak = 2, as 1 < 2.
The array after removing min peak will be  [1]
Sixth min peak = 1.
Therefore, the list of minimum peak is [6, 8, 9, 7, 2, 1].

Input: arr []= {1, 5, 3, 7, 2}
Output: [5, 7, 3, 2, 1]
Explanation:
First min peak = 5, as 1 < 5 > 3.
The array after removing min peak will be [1, 3, 7, 2]
Second min peak = 7,  as 3 < 7 > 2.
The array after removing min peak will be [1, 3, 2]
Third min peak = 3, as 1 < 3 > 2.
The array after removing min peak will be [1, 2]
Fourth min peak = 2, as 1 < 2.
The array after removing min peak will be  [1]
Fifth min peak = 1.
Therefore, the list of minimum peak is [5, 7, 3, 2, 1]

Approach: The idea is to find the minimum peak element of the array by iterating over the array using two nested loops, where the outer loop points to the current element and the inner loop execute to find the index of min peak element, remove that peak element from the array and store the current peak element in the resultant list. After completing the above steps, print all the minimum peak elements stored in the list.

Below is the implementation of the above approach:

## C++

 `// C++ program for the above approach``#include``using` `namespace` `std;` `// Function to return the list of``// minimum peak elements``void` `minPeaks(vector<``int``>list)``{``    ` `    ``// Length of original list``    ``int` `n = list.size();``    ` `    ``// Initialize resultant list``    ``vector<``int``>result;` `    ``// Traverse each element of list``    ``for``(``int` `i = 0; i < n; i++)``    ``{``        ``int` `min = INT_MAX;``        ``int` `index = -1;` `        ``// Length of original list after``        ``// removing the peak element``        ``int` `size = list.size();` `        ``// Traverse new list after removal``        ``// of previous min peak element``        ``for``(``int` `j = 0; j < size; j++)``        ``{``            ` `            ``// Update min and index,``            ``// if first element of``            ``// list > next element``            ``if` `(j == 0 && j + 1 < size)``            ``{``                ``if` `(list[j] > list[j + 1] &&``                        ``min > list[j])``                ``{``                    ``min = list[j];``                    ``index = j;``                ``}``            ``}` `            ``else` `if` `(j == size - 1 &&``                     ``j - 1 >= 0)``            ``{``                ` `                ``// Update min and index,``                ``// if last element of``                ``// list > previous one``                ``if` `(list[j] > list[j - 1] &&``                        ``min > list[j])``                ``{``                    ``min = list[j];``                    ``index = j;``                ``}``            ``}` `            ``// Update min and index, if``            ``// list has single element``            ``else` `if` `(size == 1)``            ``{``                ``min = list[j];``                ``index = j;``            ``}` `            ``// Update min and index,``            ``// if current element >``            ``// adjacent elements``            ``else` `if` `(list[j] > list[j - 1] &&``                     ``list[j] > list[j + 1] &&``                         ``min > list[j])``            ``{``                ``min = list[j];``                ``index = j;``            ``}``        ``}` `        ``// Remove current min peak``        ``// element from list``        ``list.erase(list.begin() + index);` `        ``// Insert min peak into``        ``// resultant list``        ``result.push_back(min);``    ``}``    ` `    ``// Print resultant list``    ``cout << ``"["``;``    ``for``(``int` `i = 0; i < result.size(); i++)``    ``{``        ``cout << result[i] << ``", "``;``    ``}``    ``cout << ``"]"``;``}` `// Driver Code``int` `main()``{``    ` `    ``// Given array arr[]``    ``vector<``int``> arr = { 1, 9, 7, 8, 2, 6 };` `    ``// Function call``    ``minPeaks(arr);``}` `// This code is contributed by bikram2001jha`

## Java

 `// Java program for the above approach` `import` `java.util.*;``import` `java.lang.*;` `class` `GFG {` `    ``// Function to return the list of``    ``// minimum peak elements``    ``static` `void``    ``minPeaks(ArrayList list)``    ``{` `        ``// Length of original list``        ``int` `n = list.size();` `        ``// Initialize resultant list``        ``ArrayList result``            ``= ``new` `ArrayList<>();` `        ``// Traverse each element of list``        ``for` `(``int` `i = ``0``; i < n; i++) {` `            ``int` `min = Integer.MAX_VALUE;``            ``int` `index = -``1``;` `            ``// Length of original list after``            ``// removing the peak element``            ``int` `size = list.size();` `            ``// Traverse new list after removal``            ``// of previous min peak element``            ``for` `(``int` `j = ``0``; j < size; j++) {` `                ``// Update min and index,``                ``// if first element of``                ``// list > next element``                ``if` `(j == ``0` `&& j + ``1` `< size) {` `                    ``if` `(list.get(j) > list.get(j + ``1``)``                        ``&& min > list.get(j)) {``                        ``min = list.get(j);``                        ``index = j;``                    ``}``                ``}` `                ``else` `if` `(j == size - ``1``                         ``&& j - ``1` `>= ``0``) {` `                    ``// Update min and index,``                    ``// if last element of``                    ``// list > previous one``                    ``if` `(list.get(j)``                            ``> list.get(j - ``1``)``                        ``&& min``                               ``> list.get(j)) {``                        ``min = list.get(j);``                        ``index = j;``                    ``}``                ``}` `                ``// Update min and index, if``                ``// list has single element``                ``else` `if` `(size == ``1``) {` `                    ``min = list.get(j);``                    ``index = j;``                ``}` `                ``// Update min and index,``                ``// if current element >``                ``// adjacent elements``                ``else` `if` `(list.get(j)``                             ``> list.get(j - ``1``)``                         ``&& list.get(j)``                                ``> list.get(j + ``1``)``                         ``&& min``                                ``> list.get(j)) {` `                    ``min = list.get(j);``                    ``index = j;``                ``}``            ``}` `            ``// Remove current min peak``            ``// element from list``            ``list.remove(index);` `            ``// Insert min peak into``            ``// resultant list``            ``result.add(min);``        ``}` `        ``// Print resultant list``        ``System.out.println(result);``    ``}` `    ``// Driver Code``    ``public` `static` `void` `main(String[] args)``    ``{``        ``// Given array arr[]``        ``ArrayList arr = ``new` `ArrayList<>(``            ``Arrays.asList(``1``, ``9``, ``7``, ``8``, ``2``, ``6``));` `        ``// Function Call``        ``minPeaks(arr);``    ``}``}`

## Python3

 `# Python3 program for``# the above approach``import` `sys` `# Function to return the list of``# minimum peak elements``def` `minPeaks(list1):` `    ``# Length of original list``    ``n ``=` `len``(list1)``    ` `    ``# Initialize resultant list``    ``result ``=` `[]` `    ``# Traverse each element of list``    ``for` `i ``in` `range` `(n):``        ``min` `=` `sys.maxsize``        ``index ``=` `-``1` `        ``# Length of original list``        ``# after removing the peak``        ``# element``        ``size ``=` `len``(list1)` `        ``# Traverse new list after removal``        ``# of previous min peak element``        ``for` `j ``in` `range` `(size):``            ` `            ``# Update min and index,``            ``# if first element of``            ``# list > next element``            ``if` `(j ``=``=` `0` `and` `j ``+` `1` `< size):``      ` `                ``if` `(list1[j] > list1[j ``+` `1``] ``and``                    ``min` `> list1[j]):``                    ``min` `=` `list1[j];``                    ``index ``=` `j;``               ` `            ``elif` `(j ``=``=` `size ``-` `1` `and``                  ``j ``-` `1` `>``=` `0``):``            ` `                ``# Update min and index,``                ``# if last element of``                ``# list > previous one``                ``if` `(list1[j] > list1[j ``-` `1``] ``and``                    ``min` `> list1[j]):``                    ``min` `=` `list1[j]``                    ``index ``=` `j``              ` `            ``# Update min and index, if``            ``# list has single element``            ``elif` `(size ``=``=` `1``):``                ``min` `=` `list1[j]``                ``index ``=` `j``       ` `            ``# Update min and index,``            ``# if current element >``            ``# adjacent elements``            ``elif` `(list1[j] > list1[j ``-` `1``] ``and``                  ``list1[j] > list1[j ``+` `1``] ``and``                  ``min` `> list1[j]):``                ``min` `=` `list1[j]``                ``index ``=` `j``     ` `        ``# Remove current min peak``        ``# element from list``        ``list1.pop(index)` `        ``# Insert min peak into``        ``# resultant list``        ``result.append(``min``)``   ` `    ``# Print resultant list``    ``print` `(result)` `# Driver Code``if` `__name__ ``=``=` `"__main__"``:``    ` `    ``# Given array arr[]``    ``arr ``=` `[``1``, ``9``, ``7``, ``8``, ``2``, ``6``]` `    ``# Function call``    ``minPeaks(arr)``  ` `# This code is contributed by Chitranayal`

## C#

 `// C# program for``// the above approach``using` `System;``using` `System.Collections.Generic;``class` `GFG{` `// Function to return the list of``// minimum peak elements``static` `void` `minPeaks(List<``int``> list)``{``  ``// Length of original list``  ``int` `n = list.Count;` `  ``// Initialize resultant list``  ``List<``int``> result = ``new` `List<``int``>();` `  ``// Traverse each element of list``  ``for` `(``int` `i = 0; i < n; i++)``  ``{``    ``int` `min = ``int``.MaxValue;``    ``int` `index = -1;` `    ``// Length of original list after``    ``// removing the peak element``    ``int` `size = list.Count;` `    ``// Traverse new list after removal``    ``// of previous min peak element``    ``for` `(``int` `j = 0; j < size; j++)``    ``{``      ``// Update min and index,``      ``// if first element of``      ``// list > next element``      ``if` `(j == 0 && j + 1 < size)``      ``{``        ``if` `(list[j] > list[j + 1] &&``            ``min > list[j])``        ``{``          ``min = list[j];``          ``index = j;``        ``}``      ``}` `      ``else` `if` `(j == size - 1 && j - 1 >= 0)``      ``{``        ``// Update min and index,``        ``// if last element of``        ``// list > previous one``        ``if` `(list[j] > list[j - 1] &&``            ``min > list[j])``        ``{``          ``min = list[j];``          ``index = j;``        ``}``      ``}` `      ``// Update min and index, if``      ``// list has single element``      ``else` `if` `(size == 1)``      ``{``        ``min = list[j];``        ``index = j;``      ``}` `      ``// Update min and index,``      ``// if current element >``      ``// adjacent elements``      ``else` `if` `(list[j] > list[j - 1] &&``               ``list[j] > list[j + 1] &&``               ``min > list[j])``      ``{``        ``min = list[j];``        ``index = j;``      ``}``    ``}` `    ``// Remove current min peak``    ``// element from list``    ``list.RemoveAt(index);` `    ``// Insert min peak into``    ``// resultant list``    ``result.Add(min);``  ``}` `  ``// Print resultant list``  ``for` `(``int` `i = 0; i < result.Count; i++)``    ``Console.Write(result[i] +``", "``);``}` `// Driver Code``public` `static` `void` `Main(String[] args)``{``  ``// Given array []arr``  ``List<``int``> arr = ``new` `List<``int``>{1, 9, 7,``                                ``8, 2, 6};` `  ``// Function Call``  ``minPeaks(arr);``}``}` `// This code is contributed by 29AjayKumar`

## Javascript

 ``
Output:
`[6, 8, 9, 7, 2, 1]`

Time Complexity: O(N2)
Auxiliary Space: O(N)

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