Next Smaller Element in C++

The next smaller element is the element that is the first smaller element after it. Let's see an example.

arr = [1, 2, 3, 5, 4]

The next smaller element for 5 is 4 and the next smaller element for elements 1, 2, 3 is -1 as there is no smaller element after them.

Algorithm

• Initialise the array with random numbers

• Initialise a stack.

• Add first element to the stack.

• Iterate through the element of the array.

  • If the stack is empty, add the current element to the stack.

  • While the current element is smaller than the top element of the stack.

    • Print the top element with the next smaller element as current element.

    • Pop the top element.

  • Add the element to the stack.

• While stack is not empty.

  • Print the elements with next smaller element as -1.

Implementation

Following is the implementation of the above algorithm in C++

#include <bits/stdc++.h>
using namespace std;
void nextSmallerElements(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() && s.top() > arr[i]) {
         cout << s.top() << " -> " << arr[i] << endl;
         s.pop();
      }
      s.push(arr[i]);
   }
   while (!s.empty()) {
      cout << s.top() << " -> " << -1 << endl;
      s.pop();
   }
}
int main() {
   int arr[] = { 5, 4, 3, 2, 1 };
   int n = 5;
   nextSmallerElements(arr, n);
   return 0;
}

Output

If you run the above code, then you will get the following result.

1 -> 2
2 -> 3
3 -> 4
4 -> 5
5 -> -1