next_greatest_element_stack.cpp

#include <bits/stdc++.h> 
using namespace std; 
  
void printNGE(int arr[], int n) { 
  stack < int > s; 
  
  s.push(arr[0]); 
  
  for (int i = 1; i < n; i++) { 
  
    if (s.empty()) { 
      s.push(arr[i]); 
      continue; 
    } 
  
    while (s.empty() == false && s.top() < arr[i]) 
    {          
        cout << s.top() << " --> " << arr[i] << endl; 
        s.pop(); 
    } 
  
    s.push(arr[i]); 
  } 
  
  while (s.empty() == false) { 
    cout << s.top() << " --> " << -1 << endl; 
    s.pop(); 
  } 
} 
  
/* Driver program */
int main() { 
  int arr[] = {11, 13, 21, 3}; 
  int n = sizeof(arr) / sizeof(arr[0]); 
  printNGE(arr, n); 
  return 0; 
}