NextGreaterElement.cs

using System;
using System.Collections.Generic;

namespace Algorithms.Stack
{
    /// <summary>
    ///     For each element in an array, the utility finds the next greater element on the right side using a stack.
    /// @author Mohit Singh. <a href="https://github.com/mohit-gogitter">mohit-gogitter</a>
    /// </summary>
    public class NextGreaterElement
    {
        /// <summary>
        /// Finds the next greater element for each element in the input array.
        /// The next greater element for an element x is the first element greater than x to its right.
        /// If there is no greater element, -1 is returned for that element.
        /// </summary>
        /// <param name="nums">The array of integers to find the next greater elements for.</param>
        /// <returns>An array where each index contains the next greater element of the corresponding element in the input array, or -1 if no such element exists.</returns>
        /// <exception cref="ArgumentNullException">Thrown when the input array is null.</exception>
        public int[] FindNextGreaterElement(int[] nums)
        {
            int[] result = new int[nums.Length];
            Stack<int> stack = new Stack<int>();

            // Initialize all elements in the result array to -1
            for (int i = 0; i < nums.Length; i++)
            {
                result[i] = -1;
            }

            for (int i = 0; i < nums.Length; i++)
            {
                // While the stack is not empty and the current element is greater than the element
                // corresponding to the index stored at the top of the stack
                while (stack.Count > 0 && nums[i] > nums[stack.Peek()])
                {
                    int index = stack.Pop();
                    result[index] = nums[i]; // Set the next greater element
                }

                stack.Push(i); // Push current index to stack
            }

            return result;
        }
    }
}