通过删除大于其左侧所有元素的元素来减少数组

给定一个由N 个整数组成的数组arr[] ，任务是从给定数组中删除元素，如果它左边的元素小于它。继续从数组中删除元素，直到没有元素具有较小的相邻左元素为止。打印上述操作后的结果数组。

例子：

Input: arr[] = {2, 4, 1, 3, 4} 
Output: 2 1 
Explanation: 
Since 4 is greater than 2 remove 4, and arr become {2, 1, 3, 4}. 
Now 3 is greater than 1 so remove 3 and arr become {2, 1, 4}. 
Now 4 is greater than 1 so remove 4 and arr become {2, 1}. 
Now no elements satisfy the removing criteria so the resultant array is {2, 1}.

Input: arr[] = {5, 4, 3, 2, 1} 
Output: 5 4 3 2 1 

方法：思路是使用归并排序的概念。

1. 将输入数组划分为子数组，直到每个子数组的大小变为 1。
2. 开始合并元素。
3. 合并时，从左子数组中删除元素，直到它最右边的元素，其值大于右子数组最左边的元素。
4. 在每个合并步骤中重复上述步骤，这样所有值较小的元素都已被删除。
5. 最后打印结果数组。

下面是上述方法的实现：

C++

// C++ program for the above approach
#include
using namespace std;
 
// Function to implement merging of arr[]
vector merge(vector x, vector y)
{
    for(auto i : y)
    {
        if (x[x.size() - 1] > i)
            x.push_back(i);
    }
    return x;
}
 
// Function to delete all elements which
// satisfy the condition A[i] > A[i-1]
vector mergeDel(vector l)
{
     
    // Divide array into its subarray
    if (l.size() == 1)
        return l;
 
    int m = l.size() / 2;
     
    vector temp1 = {l.begin() + 0,
                         l.begin() + m};
    vector temp2 = {l.begin() + m,
                         l.end()};
     
    // Getting back merged array with all 
    // its right element greater than
    // left one.
    return merge(mergeDel(temp1),
                 mergeDel(temp2));
}
 
// Driver Code
int main()
{
     
    // Given array arr[]
    vector arr({ 5, 4, 3, 2, 1 });
    vector ans = mergeDel(arr);
     
    cout << "[ ";
    for(auto x: ans)
        cout << x << ", ";
         
    cout << "]";
}
 
// This code is contributed by SURENDRA_GANGWAR

Java

// Java program for the above approach
import java.util.ArrayList;
import java.util.Arrays;
 
class GFG{
     
// Function to implement merging of arr[]
static ArrayList merge(ArrayList x,
                                ArrayList y)
{
    for(Integer i : y)
    {
        if (x.get(x.size() - 1) > i)
            x.add(i);
    }
    return x;
}
 
// Function to delete all elements which
// satisfy the condition A[i] > A[i-1]
static ArrayList mergeDel(ArrayList l)
{
     
    // Divide array into its subarray
    if (l.size() == 1)
        return l;
 
    int m = l.size() / 2;
 
    ArrayList temp1 = new ArrayList<>(
        l.subList(0, m));
    ArrayList temp2 = new ArrayList<>(
        l.subList(m, l.size()));
 
    // Getting back merged array with all
    // its right element greater than
    // left one.
    return merge(mergeDel(temp1), mergeDel(temp2));
}
 
// Driver Code
public static void main(String[] args)
{
     
    // Given array arr[]
    Integer[] ar = { 5, 4, 3, 2, 1 };
     
    ArrayList arr = new ArrayList<>(
        Arrays.asList(ar));
    ArrayList ans = mergeDel(arr);
     
    System.out.print("[ ");
    for(Integer x : ans)
        System.out.print(x + ", ");
         
    System.out.println("]");
}
}
 
// This code is contributed by sanjeev2552

Python3

# Python3 program for the above approach
 
# Function to delete all elements which
# satisfy the condition A[i] > A[i-1]
def mergeDel(l):
 
    # Divide array into its subarray
    if len(l) == 1:
 
        return l
 
    m = int( len(l) / 2)
 
    # Getting back merged array with all
    # its right element greater than left one.
    return merge(mergeDel(l[ 0 : m ]),
                 mergeDel(l[ m : len(l)]) )
 
 
# Function to implement merging of arr[]
def merge(x, y):
 
    for i in y:
 
        if x[-1] > i :
 
            x = x + [i]
 
    return x
   
# Driver Code
 
# Function defination for main()
def main():
 
# Given array arr[]
    arr = [5, 4, 3, 2, 1]
    print(mergeDel(arr))
 
main()

C#

// C# program for the above approach
using System;
using System.Collections.Generic;
 
class GFG{
 
// Function to implement merging of arr[]
static List merge(List x, List y)
{
    foreach(int i in y)
    {
        if (x[x.Count - 1] > i)
            x.Add(i);
    }
    return x;
}
 
// Function to delete all elements which
// satisfy the condition A[i] > A[i-1]
static List mergeDel(List l)
{
     
    // Divide array into its subarray
    if (l.Count == 1)
        return l;
 
    int m = l.Count / 2;
 
    List temp1 = l.GetRange(0, m);
    List temp2 = l.GetRange(m, l.Count - m);
 
    // Getting back merged array with all
    // its right element greater than
    // left one.
    return merge(mergeDel(temp1),
                 mergeDel(temp2));
}
 
// Driver Code
public static void Main(string[] args)
{
     
    // Given array arr[]
    List arr = new List{ 5, 4, 3, 2, 1 };
 
    List ans = mergeDel(arr);
 
    Console.Write("[ ");
    foreach(int x in ans)
        Console.Write(x + ", ");
 
    Console.Write("]");
}
}
 
// This code is contributed by chitranayal

输出：

[5, 4, 3, 2, 1]

时间复杂度： O(N*log N)
辅助空间： O(1)

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