ArithmeticExpressionTree.cs

using System;
using System.Collections.Generic;

namespace ArithmeticExpressionsTree
{
    public struct TreeNode
    {
        public TreeNode(Int32 parentIndex, String value)
        {
            ParentIndex = parentIndex;
            Value = value;
        }

        public readonly Int32 ParentIndex;
        public readonly String Value;
    }

    public class ArithmeticExpressionTree
    {
        public ArithmeticExpressionTree(String rootValue)
        {
            nodes = new List<TreeNode> {new TreeNode(-1, rootValue)};
        }

        public Int32 AddNode(Int32 parentNodeIndex, String value)
        {
            nodes.Add(new TreeNode(parentNodeIndex, value));
            return nodes.Count - 1;
        }

        public TreeNode GetNode(Int32 index)
        {
            return nodes[index];
        }

        public List<Int32> GetAllChildrenIndexes(Int32 nodeIndex)
        {
            var result = new List<Int32>();
            for (Int32 i = 0; i < nodes.Count; i++)
            {
                if (nodes[i].ParentIndex == nodeIndex)
                    result.Add(i);
            }

            return result;
        }

        public List<TreeNode> GetAllChildren(Int32 nodeIndex)
        {
            return nodes.FindAll(node => node.ParentIndex == nodeIndex);
        }

        public Int32 RootIndex => 0;
        public TreeNode Root => nodes[0];

        private List<TreeNode> nodes;
    }
}