Task: Sum to 100
C#: using System;
using System.Collections.Generic;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        
        var expressionsPlus = Enumerable.Range(0, (int)Math.Pow(3, 8)).AsParallel().Select(i => new Expression(i, 1));
        
        var expressionsMinus = Enumerable.Range(0, (int)Math.Pow(3, 8)).AsParallel().Select(i => new Expression(i, -1));
        var expressions = expressionsPlus.Concat(expressionsMinus);
        var results = new Dictionary<int, List<Expression>>();
        foreach (var e in expressions)
        {
            if (results.Keys.Contains(e.Value))
                results[e.Value].Add(e);
            else
                results[e.Value] = new List<Expression>() { e };
        }
        Console.WriteLine("Show all solutions that sum to 100");
        foreach (Expression e in results[100])
            Console.WriteLine("  " + e);
        Console.WriteLine("Show the sum that has the maximum number of solutions (from zero to infinity)");
        var summary = results.Keys.Select(k => new Tuple<int, int>(k, results[k].Count));
        var maxSols = summary.Aggregate((a, b) => a.Item2 > b.Item2 ? a : b);
        Console.WriteLine("  The sum " + maxSols.Item1 + " has " + maxSols.Item2 + " solutions.");
        Console.WriteLine("Show the lowest positive sum that can't be expressed (has no solutions), using the rules for this task");
        var lowestPositive = Enumerable.Range(1, int.MaxValue).First(x => !results.Keys.Contains(x));
        Console.WriteLine("  " + lowestPositive);
        Console.WriteLine("Show the ten highest numbers that can be expressed using the rules for this task (extra credit)");
        var highest = from k in results.Keys
                      orderby k descending
                      select k;
        foreach (var x in highest.Take(10))
            Console.WriteLine("  " + x);
    }
}
public enum Operations { Plus, Minus, Join };
public class Expression
{
    protected Operations[] Gaps;
    
    
    public int Value; 
    protected int _one;
    
    public Expression(int serial, int one)
    {
        _one = one;
        Gaps = new Operations[8];
        
        int divisor = 2187; 
        int times;
        for (int i = 0; i < 8; i++)
        {
            times = Math.DivRem(serial, divisor, out serial);
            divisor /= 3;
            if (times == 0)
                Gaps[i] = Operations.Join;
            else if (times == 1)
                Gaps[i] = Operations.Minus;
            else
                Gaps[i] = Operations.Plus;
        }
        
        
        Value = Evaluate();
    }
    public override string ToString()
    {
        string ret = _one.ToString();
        for (int i = 0; i < 8; i++)
        {
            switch (Gaps[i])
            {
                case Operations.Plus:
                    ret += "+";
                    break;
                case Operations.Minus:
                    ret += "-";
                    break;
            }
            ret += (i + 2);
        }
        return ret;
    }
    private int Evaluate()
        
    {
        var numbers = new int[9];
        int nc = 0;
        var operations = new List<Operations>();
        int a = 1;
        for (int i = 0; i < 8; i++)
        {
            if (Gaps[i] == Operations.Join)
                a = a * 10 + (i + 2);
            else
            {
                if (a > 0)
                {
                    if (nc == 0)
                        a *= _one;
                    numbers[nc++] = a;
                    a = i + 2;
                }
                operations.Add(Gaps[i]);
            }
        }
        if (nc == 0)
            a *= _one;
        numbers[nc++] = a;
        int ni = 0;
        int left = numbers[ni++];
        foreach (var operation in operations)
        {
            int right = numbers[ni++];
            if (operation == Operations.Plus)
                left = left + right;
            else
                left = left - right;
        }
        return left;
    }
}

Java: 
package rosettacode;

import java.io.PrintStream;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

public class SumTo100 implements Runnable {

    public static void main(String[] args) {
        new SumTo100().run();
    }

    void print(int givenSum) {
        Expression expression = new Expression();
        for (int i = 0; i < Expression.NUMBER_OF_EXPRESSIONS; i++, expression.next()) {
            if (expression.toInt() == givenSum) {
                expression.print();
            }
        }
    }

    void comment(String commentString) {
        System.out.println();
        System.out.println(commentString);
        System.out.println();
    }

    @Override
    public void run() {
        final Stat stat = new Stat();

        comment("Show all solutions that sum to 100");
        final int givenSum = 100;
        print(givenSum);

        comment("Show the sum that has the maximum number of solutions");
        final int maxCount = Collections.max(stat.sumCount.keySet());
        int maxSum;
        Iterator<Integer> it = stat.sumCount.get(maxCount).iterator();
        do {
            maxSum = it.next();
        } while (maxSum < 0);
        System.out.println(maxSum + " has " + maxCount + " solutions");

        comment("Show the lowest positive number that can't be expressed");
        int value = 0;
        while (stat.countSum.containsKey(value)) {
            value++;
        }
        System.out.println(value);

        comment("Show the ten highest numbers that can be expressed");
        final int n = stat.countSum.keySet().size();
        final Integer[] sums = stat.countSum.keySet().toArray(new Integer[n]);
        Arrays.sort(sums);
        for (int i = n - 1; i >= n - 10; i--) {
            print(sums[i]);
        }
    }

    private static class Expression {

        private final static int NUMBER_OF_DIGITS = 9;
        private final static byte ADD = 0;
        private final static byte SUB = 1;
        private final static byte JOIN = 2;

        final byte[] code = new byte[NUMBER_OF_DIGITS];
        final static int NUMBER_OF_EXPRESSIONS = 2 * 3 * 3 * 3 * 3 * 3 * 3 * 3 * 3;

        Expression next() {
            for (int i = 0; i < NUMBER_OF_DIGITS; i++) {
                if (++code[i] > JOIN) {
                    code[i] = ADD;
                } else {
                    break;
                }
            }
            return this;
        }

        int toInt() {
            int value = 0;
            int number = 0;
            int sign = (+1);
            for (int digit = 1; digit <= 9; digit++) {
                switch (code[NUMBER_OF_DIGITS - digit]) {
                    case ADD:
                        value += sign * number;
                        number = digit;
                        sign = (+1);
                        break;
                    case SUB:
                        value += sign * number;
                        number = digit;
                        sign = (-1);
                        break;
                    case JOIN:
                        number = 10 * number + digit;
                        break;
                }
            }
            return value + sign * number;
        }

        @Override
        public String toString() {
            StringBuilder s = new StringBuilder(2 * NUMBER_OF_DIGITS + 1);
            for (int digit = 1; digit <= NUMBER_OF_DIGITS; digit++) {
                switch (code[NUMBER_OF_DIGITS - digit]) {
                    case ADD:
                        if (digit > 1) {
                            s.append('+');
                        }
                        break;
                    case SUB:
                        s.append('-');
                        break;
                }
                s.append(digit);
            }
            return s.toString();
        }

        void print() {
            print(System.out);
        }

        void print(PrintStream printStream) {
            printStream.format("%9d", this.toInt());
            printStream.println(" = " + this);
        }
    }

    private static class Stat {

        final Map<Integer, Integer> countSum = new HashMap<>();
        final Map<Integer, Set<Integer>> sumCount = new HashMap<>();

        Stat() {
            Expression expression = new Expression();
            for (int i = 0; i < Expression.NUMBER_OF_EXPRESSIONS; i++, expression.next()) {
                int sum = expression.toInt();
                countSum.put(sum, countSum.getOrDefault(sum, 0) + 1);
            }
            for (Map.Entry<Integer, Integer> entry : countSum.entrySet()) {
                Set<Integer> set;
                if (sumCount.containsKey(entry.getValue())) {
                    set = sumCount.get(entry.getValue());
                } else {
                    set = new HashSet<>();
                }
                set.add(entry.getKey());
                sumCount.put(entry.getValue(), set);
            }
        }
    }
}