Task: Sum to 100
C++: 
#include <iostream>
#include <iomanip>
#include <algorithm>
#include <string>
#include <set>
#include <map>

using namespace std;

class Expression{
    private:
        enum { NUMBER_OF_DIGITS = 9 }; 
        enum Op { ADD, SUB, JOIN };
        int code[NUMBER_OF_DIGITS];
    public:
        static const int NUMBER_OF_EXPRESSIONS;
        Expression(){
            for ( int i = 0; i < NUMBER_OF_DIGITS; i++ )
                code[i] = ADD;
        }
        Expression& operator++(int){ 
            for ( int i = 0; i < NUMBER_OF_DIGITS; i++ )
                if ( ++code[i] > JOIN ) code[i] = ADD; 
                else break;
            return *this;        
        }
        operator int() const{
            int value = 0, number = 0, 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;
        }
        operator string() const{
            string s;
            for ( int digit = 1; digit <= NUMBER_OF_DIGITS; digit++ ){
                switch( code[NUMBER_OF_DIGITS - digit] ){
                    case ADD: if ( digit > 1 ) s.push_back('+'); break;
                    case SUB:                  s.push_back('-'); break;
                }
                s.push_back('0' + digit);
            }
            return s;
        }
};
const int Expression::NUMBER_OF_EXPRESSIONS = 2 * 3*3*3*3 * 3*3*3*3;

ostream& operator<< (ostream& os, Expression& ex){
    ios::fmtflags oldFlags(os.flags());
    os << setw(9) << right << static_cast<int>(ex)    << " = " 
       << setw(0) << left  << static_cast<string>(ex) << endl;
    os.flags(oldFlags);
    return os;
}

struct Stat{
    map<int,int> countSum;
    map<int, set<int> > sumCount;
    Stat(){
        Expression expression;
        for ( int i = 0; i < Expression::NUMBER_OF_EXPRESSIONS; i++, expression++ )
            countSum[expression]++;
        for ( auto it = countSum.begin(); it != countSum.end(); it++ )
            sumCount[it->second].insert(it->first);
    }
};

void print(int givenSum){
    Expression expression;
    for ( int i = 0; i < Expression::NUMBER_OF_EXPRESSIONS; i++, expression++ )
        if ( expression == givenSum ) 
            cout << expression;
}

void comment(string commentString){
    cout << endl << commentString << endl << endl;
}

int main(){
    Stat stat;

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

    comment( "Show the sum that has the maximum number of solutions" );
    auto maxi = max_element(stat.sumCount.begin(),stat.sumCount.end());
    auto it = maxi->second.begin(); 
    while ( *it < 0 ) it++;
    cout << static_cast<int>(*it) << " has " << maxi->first << " solutions" << endl;

    comment( "Show the lowest positive number that can't be expressed" );
    int value = 0; 
    while(stat.countSum.count(value) != 0) value++;
    cout << value << endl;

    comment( "Show the ten highest numbers that can be expressed" );
    auto rit = stat.countSum.rbegin();
    for ( int i = 0; i < 10; i++, rit++ ) print(rit->first);

    return 0;
}

Go: package main

import (
	"fmt"
	"sort"
)

const pow3_8 = 3 * 3 * 3 * 3 * 3 * 3 * 3 * 3 
const pow3_9 = 3 * pow3_8                    
const maxExprs = 2 * pow3_8                  

type op uint8

const (
	Add  op = iota 
	Sub            
	Join           
)




type code uint16


func (c code) evaluate() (sum int) {
	num, pow := 0, 1
	for k := 9; k >= 1; k-- {
		num += pow * k
		switch op(c % 3) {
		case Add:
			sum += num
			num, pow = 0, 1
		case Sub:
			sum -= num
			num, pow = 0, 1
		case Join:
			pow *= 10
		}
		c /= 3
	}
	return sum
}

func (c code) String() string {
	buf := make([]byte, 0, 18)
	a, b := code(pow3_9), code(pow3_8)
	for k := 1; k <= 9; k++ {
		switch op((c % a) / b) {
		case Add:
			if k > 1 {
				buf = append(buf, '+')
			}
		case Sub:
			buf = append(buf, '-')
		}
		buf = append(buf, '0'+byte(k))
		a, b = b, b/3
	}
	return string(buf)
}

type sumCode struct {
	sum  int
	code code
}
type sumCodes []sumCode

type sumCount struct {
	sum   int
	count int
}
type sumCounts []sumCount


func (p sumCodes) Len() int            { return len(p) }
func (p sumCodes) Swap(i, j int)       { p[i], p[j] = p[j], p[i] }
func (p sumCodes) Less(i, j int) bool  { return p[i].sum < p[j].sum }
func (p sumCounts) Len() int           { return len(p) }
func (p sumCounts) Swap(i, j int)      { p[i], p[j] = p[j], p[i] }
func (p sumCounts) Less(i, j int) bool { return p[i].count > p[j].count }


func (sc sumCode) String() string {
	return fmt.Sprintf("% 10d = %v", sc.sum, sc.code)
}
func (sc sumCount) String() string {
	return fmt.Sprintf("% 10d has %d solutions", sc.sum, sc.count)
}

func main() {
	
	expressions := make(sumCodes, 0, maxExprs/2)
	counts := make(sumCounts, 0, 1715)
	for c := code(0); c < maxExprs; c++ {
		
		
		
		sum := c.evaluate()
		if sum >= 0 {
			expressions = append(expressions, sumCode{sum, c})
		}
	}
	sort.Sort(expressions)

	
	sc := sumCount{expressions[0].sum, 1}
	for _, e := range expressions[1:] {
		if e.sum == sc.sum {
			sc.count++
		} else {
			counts = append(counts, sc)
			sc = sumCount{e.sum, 1}
		}
	}
	counts = append(counts, sc)
	sort.Sort(counts)

	

	fmt.Println("All solutions that sum to 100:")
	i := sort.Search(len(expressions), func(i int) bool {
		return expressions[i].sum >= 100
	})
	for _, e := range expressions[i:] {
		if e.sum != 100 {
			break
		}
		fmt.Println(e)
	}

	fmt.Println("\nThe positive sum with maximum number of solutions:")
	fmt.Println(counts[0])

	fmt.Println("\nThe lowest positive number that can't be expressed:")
	s := 1
	for _, e := range expressions {
		if e.sum == s {
			s++
		} else if e.sum > s {
			fmt.Printf("% 10d\n", s)
			break
		}
	}

	fmt.Println("\nThe ten highest numbers that can be expressed:")
	for _, e := range expressions[len(expressions)-10:] {
		fmt.Println(e)
	}
}