3Given an array of distinct elements, print the closest greater element for every element. The closest greater element for an element x is the smallest element on the right side of x in array which is greater than x. Elements for which no greater element exist, consider next greater element as -1.
Examples:
Input: arr[] = {4, 5, 2, 25}
Output:
Element NGE
4 --> 5
5 --> 25
2 --> 25
25 --> -1
Input: arr[] = {4, 10, 7}
Output:
Element NGE
4 --> 7
10 --> -1
7 --> -1
Approach: In this post, we will be discussing how to find the Next Greater Element using C++ STL(set).
Finding the smallest greater element on the right side will be like finding the first greater element of the current element in a list that is sorted.
Consider example 1, The sorted list would look like 2, 4, 5, 25.
Here for element 4, the greater element is 5 as it is next to it, so we print 5 and remove 4 because it would not be greater to the other elements since it is no longer on anyone's right.
Similarly, for 5 it is 25 and we remove 5 from the list, as 5 will not be on the right side of 2 or 25, so it can be deleted.
Given below are the steps to find the Next Greater Element of every index element.
- Insert all the elements in a Set, it will store all the elements in an increasing order.
- Iterate on the array of elements, and for each index, find the upper_bound of the current index element. The upper_bound() returns an iterator which can point to the following position.
- If the iterator is pointing to a position past the last element, then there exists no NGE to the current index element.
- If the iterator points to a position referring to an element, then that element is the NGE to the current index element.
- Find the position of current index element at every traversal and remove it from the set using >lower_bound() and erase() functions of set.
Implementation:
// C++ program to print the
// NGE's of array elements using
// C++ STL
#include <bits/stdc++.h>
using namespace std;
// Function to print the NGE
void printNGE(int a[], int n)
{
set<int> ms;
// insert in the multiset container
for (int i = 0; i < n; i++)
ms.insert(a[i]);
cout << "Element "
<< "NGE";
// traverse for all array elements
for (int i = 0; i < n; i++) {
// find the upper_bound in set
auto it = ms.upper_bound(a[i]);
// if points to the end, then
// no NGE of that element
if (it == ms.end()) {
cout << "\n " << a[i]
<< " ----> " << -1;
}
// print the element at that position
else {
cout << "\n " << a[i]
<< " ----> " << *it;
}
// find the first occurrence of
// the index element and delete it
it = ms.lower_bound(a[i]);
// delete one occurrence
// from the container
ms.erase(it);
}
}
// Driver Code
int main()
{
int a[] = { 4, 5, 2, 25 };
int n = sizeof(a) / sizeof(a[0]);
// Function call to print the NGE
printNGE(a, n);
return 0;
}
// C++ program to print the
// NGE's of array elements using
import java.util.TreeSet;
class Geeks {
// Function to print the NGE
static void printNGE(int[] a, int n)
{
// Tree Set is an ordered set used to
// store elements in a sorted manner
TreeSet<Integer> t = new TreeSet<>();
// Adding elements into the set
for (int i = 0; i < n; i++)
t.add(a[i]);
System.out.println("ELEMENT NGE");
for (int i = 0; i < n; i++) {
// If the elements does not have an upper bound
// or an element greater than it,
// higher method of TreeSet class will return NULL
if (t.higher(a[i]) == null)
System.out.println(a[i] + " ----> "
+ "-1");
// Otherwise print the upper bound of that element
else
System.out.println(a[i] + " ----> " + t.higher(a[i]));
// Remove the current element from the set
t.remove(a[i]);
}
}
// Driver code
public static void main(String[] args)
{
int a[] = { 4, 5, 2, 25 };
int n = a.length;
printNGE(a, n);
}
}
# Python3 program to print the
# NGE's of array elements
from bisect import bisect_right as upper_bound, \
bisect_left as lower_bound
# Function to print the NGE
def printNGE(a: list, n):
ms = set()
# insert in the multiset container
for i in range(n):
ms.add(a[i])
print("Element NGE", end = "")
# Required because Python sets
# are not sorted
new_arr = list(ms)
new_arr.sort()
# traverse for all array elements
for i in range(n):
# find the upper_bound in set
it = upper_bound(new_arr, a[i])
# if points to the end, then
# no NGE of that element
if (it == len(new_arr)):
print("\n %d ----> -1" % a[i], end = "")
# print the element at that position
else:
print("\n %d ----> %d" % (a[i],
new_arr[it]), end = "")
# find the first occurrence of
# the index element and delete it
it = lower_bound(new_arr, a[i])
# delete one occurrence
# from the container
new_arr.remove(new_arr[it])
# Driver Code
if __name__ == "__main__":
a = [4, 5, 2, 25]
n = len(a)
# Function call to print the NGE
printNGE(a, n)
# This code is contributed by
# sanjeev2552
// C# program for the above approach
using System;
using System.Collections.Generic;
class Geeks
{
// Function to print the NGE
static void printNGE(int[] a, int n)
{
// insert in the multiset container of array a
SortedSet<int> s = new SortedSet<int>(a);
Console.WriteLine("Element NGE");
// traverse for all array elements
for (int i = 0; i < n; i++) {
SortedSet<int>.Enumerator enumr = s.GetViewBetween(a[i] + 1, int.MaxValue).GetEnumerator();
// if points to the end, then
// no NGE of that element
if (!enumr.MoveNext()) {
Console.WriteLine($"{a[i]} ----> -1");
}
// print the element at that position
else {
Console.WriteLine($"{a[i]} ----> {enumr.Current}");
}
// delete one occurrence
// from the container
s.Remove(a[i]);
}
}
// Driver Code
public static void Main()
{
int[] a = { 4, 5, 2, 25 };
int n = a.Length;
// Function call to print the NGE
printNGE(a, n);
}
}
// This code is contributed by codebraxnzt
<script>
// Javascript program to print the
// NGE's of array elements using
// Function to print the NGE
function printNGE(a , n) {
// Tree Set is an ordered set used to
// store elements in a sorted manner
var t = new Set();
// Adding elements into the set
for (var i = 0; i < n; i++)
t.add(a[i]);
document.write("ELEMENT NGE<br/>");
for (i = 0; i < n; i++) {
// If the elements does not have an upper bound
// or an element greater than it,
// higher method of TreeSet class will return NULL
if (upper_bound(t,a[i]) == null)
document.write(a[i] + " ----> " + "-1"+"<br/>");
// Otherwise print the upper bound of that element
else
document.write(a[i] + " ----> " + upper_bound(t,a[i])+"<br/>");
// Remove the current element from the set
t.delete(a[i]);
}
}
function upper_bound(s, val)
{
let temp = [...s];
temp.sort((a, b) => b - a);
return temp[temp.indexOf(val) + 1];
}
// Driver code
var a = [ 4, 5, 2, 25 ];
var n = a.length;
printNGE(a, n);
// This code contributed by Rajput-Ji
</script>
Output
Element NGE 4 ----> 5 5 ----> 25 2 ----> 25 25 ----> -1
Complexity Analysis:
- Time Complexity: O(N*logN), as we are using a loop to traverse N times and in each traversal we are inserting to the set which will cost us logN time.
- Auxiliary Space: O(N), as we are using extra space for set ms.