I find this implementation easier to understand

using System;
using System.Collections.Generic;

public class Solution {
    public List<int> nextLargerElement(List<int> arr) {
        List<int> res = new List<int>();
        // ToDo: Write Your Code Here.
        var stack = new Stack<int>();
        var i = arr.Count - 1;

        while (i > -1) {
            if (stack.Count > 0 && stack.Peek() <= arr[i]) {
                stack.Pop();
                continue;
            }

            res.Insert(0, stack.Count > 0 ? stack.Peek() : -1);
            stack.Push(arr[i]);
            --i;
        }

        return res;
    }
}