Task: Solve a Hidato puzzle
C++: #include <iostream>
#include <sstream>
#include <iterator>
#include <vector>


using namespace std;


struct node
{
    int val;
    unsigned char neighbors;
};

class hSolver
{
public:
    hSolver()
    {
	dx[0] = -1; dx[1] = 0; dx[2] = 1; dx[3] = -1; dx[4] = 1; dx[5] = -1; dx[6] = 0; dx[7] = 1;
	dy[0] = -1; dy[1] = -1; dy[2] = -1; dy[3] = 0; dy[4] = 0; dy[5] = 1; dy[6] = 1; dy[7] = 1;
    }

    void solve( vector<string>& puzz, int max_wid )
    {
	if( puzz.size() < 1 ) return;
	wid = max_wid; hei = static_cast<int>( puzz.size() ) / wid;
	int len = wid * hei, c = 0; max = 0;
	arr = new node[len]; memset( arr, 0, len * sizeof( node ) );
	weHave = new bool[len + 1]; memset( weHave, 0, len + 1 );
		
	for( vector<string>::iterator i = puzz.begin(); i != puzz.end(); i++ )
	{
	    if( ( *i ) == "*" ) {    arr[c++].val = -1; continue; }
	    arr[c].val = atoi( ( *i ).c_str() );
	    if( arr[c].val > 0 ) weHave[arr[c].val] = true;
	    if( max < arr[c].val ) max = arr[c].val;
	    c++;
	}

	solveIt(); c = 0;
	for( vector<string>::iterator i = puzz.begin(); i != puzz.end(); i++ )
	{
	    if( ( *i ) == "." )
	    {
		ostringstream o; o << arr[c].val;
		( *i ) = o.str();
	    }
	    c++;
	}
	delete [] arr;
	delete [] weHave;
    }

private:
    bool search( int x, int y, int w )
    {
	if( w == max ) return true;

	node* n = &arr[x + y * wid];
	n->neighbors = getNeighbors( x, y );
	if( weHave[w] )
	{
	    for( int d = 0; d < 8; d++ )
	    {
		if( n->neighbors & ( 1 << d ) )
		{
		    int a = x + dx[d], b = y + dy[d];
		    if( arr[a + b * wid].val == w )
		    if( search( a, b, w + 1 ) ) return true;
		}
	    }
	    return false;
	}

	for( int d = 0; d < 8; d++ )
	{
	    if( n->neighbors & ( 1 << d ) )
	    {
		int a = x + dx[d], b = y + dy[d];
		if( arr[a + b * wid].val == 0 )
		{
		    arr[a + b * wid].val = w;
		    if( search( a, b, w + 1 ) ) return true;
		    arr[a + b * wid].val = 0;
		}
	    }
	}
	return false;
    }

    unsigned char getNeighbors( int x, int y )
    {
	unsigned char c = 0; int m = -1, a, b;
	for( int yy = -1; yy < 2; yy++ )
	    for( int xx = -1; xx < 2; xx++ )
	    {
		if( !yy && !xx ) continue;
		m++; a = x + xx, b = y + yy;
		if( a < 0 || b < 0 || a >= wid || b >= hei ) continue;
		if( arr[a + b * wid].val > -1 ) c |= ( 1 << m );
	    }
	return c;
    }

    void solveIt()
    {
	int x, y; findStart( x, y );
	if( x < 0 ) { cout << "\nCan't find start point!\n"; return; }
	search( x, y, 2 );
    }

    void findStart( int& x, int& y )
    {
	for( int b = 0; b < hei; b++ )
	    for( int a = 0; a < wid; a++ )
		if( arr[a + wid * b].val == 1 ) { x = a; y = b; return; }
	x = y = -1;
    }

    int wid, hei, max, dx[8], dy[8];
    node* arr;
    bool* weHave;
};

int main( int argc, char* argv[] )
{
    int wid;
    string p = ". 33 35 . . * * * . . 24 22 . * * * . . . 21 . . * * . 26 . 13 40 11 * * 27 . . . 9 . 1 * * * . . 18 . . * * * * * . 7 . . * * * * * * 5 ."; wid = 8;
    
    

    istringstream iss( p ); vector<string> puzz;
    copy( istream_iterator<string>( iss ), istream_iterator<string>(), back_inserter<vector<string> >( puzz ) );
    hSolver s; s.solve( puzz, wid );

    int c = 0;
    for( vector<string>::iterator i = puzz.begin(); i != puzz.end(); i++ )
    {
	if( ( *i ) != "*" && ( *i ) != "." )
	{
	    if( atoi( ( *i ).c_str() ) < 10 ) cout << "0";
	    cout << ( *i ) << " ";
	}
	else cout << "   ";
	if( ++c >= wid ) { cout << endl; c = 0; }
    }
    cout << endl << endl;
    return system( "pause" );
}


Go: package main

import (
    "fmt"
    "sort"
    "strconv"
    "strings"
)

var board [][]int
var start, given []int

func setup(input []string) {
    
    puzzle := make([][]string, len(input))
    for i := 0; i < len(input); i++ {
        puzzle[i] = strings.Fields(input[i])
    }
    nCols := len(puzzle[0])
    nRows := len(puzzle)
    list := make([]int, nRows*nCols)
    board = make([][]int, nRows+2)
    for i := 0; i < nRows+2; i++ {
        board[i] = make([]int, nCols+2)
        for j := 0; j < nCols+2; j++ {
            board[i][j] = -1
        }
    }
    for r := 0; r < nRows; r++ {
        row := puzzle[r]
        for c := 0; c < nCols; c++ {
            switch cell := row[c]; cell {
            case "_":
                board[r+1][c+1] = 0
            case ".":
                break
            default:
                val, _ := strconv.Atoi(cell)
                board[r+1][c+1] = val
                list = append(list, val)
                if val == 1 {
                    start = append(start, r+1, c+1)
                }
            }
        }
    }
    sort.Ints(list)
    given = make([]int, len(list))
    for i := 0; i < len(given); i++ {
        given[i] = list[i]
    }
}

func solve(r, c, n, next int) bool {
    if n > given[len(given)-1] {
        return true
    }

    back := board[r][c]
    if back != 0 && back != n {
        return false
    }

    if back == 0 && given[next] == n {
        return false
    }

    if back == n {
        next++
    }

    board[r][c] = n
    for i := -1; i < 2; i++ {
        for j := -1; j < 2; j++ {
            if solve(r+i, c+j, n+1, next) {
                return true
            }
        }
    }

    board[r][c] = back
    return false
}

func printBoard() {
    for _, row := range board {
        for _, c := range row {
            switch {
            case c == -1:
                fmt.Print(" . ")
            case c > 0:
                fmt.Printf("%2d ", c)
            default:
                fmt.Print("__ ")
            }
        }
        fmt.Println()
    }
}

func main() {
    input := []string{
        "_ 33 35 _ _ . . .",
        "_ _ 24 22 _ . . .",
        "_ _ _ 21 _ _ . .",
        "_ 26 _ 13 40 11 . .",
        "27 _ _ _ 9 _ 1 .",
        ". . _ _ 18 _ _ .",
        ". . . . _ 7 _ _",
        ". . . . . . 5 _",
    }
    setup(input)
    printBoard()
    fmt.Println("\nFound:")
    solve(start[0], start[1], 1, 0)
    printBoard()
}