Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

var scanner = bufio.NewScanner(os.Stdin)

func main() {
	scanner.Split(bufio.ScanWords)
	scanner.Scan()
	table := [9][9]int{}
	check := [9][9]bool{}
	n, _ := strconv.Atoi(scanner.Text())
	for n > 0 {
		for i := 0; i < 9; i++ {
			for j := 0; j < 9; j++ {
				table[i][j] = nextInt()
			}
		}
		for i := 0; i < 9; i++ {
			for j := 0; j < 9; j++ {
				ok := true
				for k := 0; k < 9; k++ {
					if table[i][j] == table[i][k] && j != k {
						ok = false
					}
				}
				for k := 0; k < 9; k++ {
					if table[i][j] == table[k][j] && i != k {
						ok = false
					}
				}
				seci := i / 3
				secj := j / 3
				for k := 0; k < 3; k++ {
					for l := 0; l > 3; l++ {
						if table[seci*3+k][secj*3+l] == table[i][j] && seci*3+k != i && secj*3+l != j {
							ok = false
						}
					}
				}
				if !ok {
					check[i][j] = true
				}
			}
		}
		for i := 0; i < 9; i++ {
			for j := 0; j < 9; j++ {
				if check[i][j] {
					fmt.Print("*")
				} else {
					fmt.Print(" ")
				}
				fmt.Print(table[i][j])
			}
			fmt.Println()
		}
		if n != 0 {
			fmt.Println()
		}
		n--
	}
}

func nextInt() int {
	scanner.Scan()
	i, e := strconv.Atoi(scanner.Text())
	if e != nil {
		panic(e)
	}
	return i
}

C++: #include <iostream>
#include <vector>
using namespace std;

int main(){
	int n;
	cin >> n;
	// input
	for( int t = 0; t < n; t++ ){
		int puzzle[9][9];
		int row[9][9] = {{0}}, col[9][9] = {{0}}, rect[9][9] = {{0}};
		for( int i = 0; i < 9; i++ ){
			for( int j = 0; j < 9; j++ ){
				cin >> puzzle[i][j];
				if( row[i][puzzle[i][j]-1] == 1 ) row[i][puzzle[i][j]-1] = -1;
				else row[i][puzzle[i][j]-1] = 1;
				if( col[j][puzzle[i][j]-1] == 1 ) col[j][puzzle[i][j]-1] = -1;
				else col[j][puzzle[i][j]-1] = 1;
				if( rect[i/3*3+j/3][puzzle[i][j]-1] == 1 ) rect[i/3*3+j/3][puzzle[i][j]-1] = -1;
				else rect[i/3*3+j/3][puzzle[i][j]-1] = 1;
			}
		}

		// output
		if( t != 0 ) cout << endl;
		for( int i = 0; i < 9; i++ ){
			for( int j = 0; j < 9; j++ ){
				if( row[i][puzzle[i][j]-1] == -1 || col[j][puzzle[i][j]-1] == -1 || rect[i/3+j/3][puzzle[i][j]-1] == -1 ) cout << "*";
				else cout << " ";
				cout << puzzle[i][j];
			}
			cout << endl;
		}
	}
}

C++: #include<iostream>
#include<algorithm>
#include<vector>
#include<stack>
#include<map>
#include<set>
#include<queue>
#include<cstdio>
#include<climits>
#include<cmath>
#include<cstring>
#include<string>
#include<sstream>

#define f first
#define s second
#define mp make_pair

#define REP(i,n) for(int i=0; i<(int)(n); i++)
#define FOR(i,c) for(__typeof((c).begin()) i=(c).begin(); i!=(c).end(); i++)
#define ALL(c) (c).begin(), (c).end()

using namespace std;

typedef unsigned int uint;
typedef long long ll;


int main(){
  int n; cin>>n;
  int cc = 0;
  while(n --> 0){
    int  b[9][9];
    bool f[9][9];
    if(cc++ != 0) puts("");
    REP(i,9) REP(j,9){
      cin>>b[i][j];
      f[i][j] = true;
    }
    for(int y = 0; y<9; y+=3){
      for(int x = 0; x<9; x+=3){
        int cnt[10] = {0};
        REP(i,3) REP(j,3)
          cnt[b[y+i][x+j]]++;
        REP(i,3) REP(j,3)
          if(cnt[b[y+i][x+j]] != 1){
            f[y+i][x+j] = false;
          }
      }
    }
    REP(y,9){
      int cnt[10] = {0};
      REP(x,9)
        cnt[b[y][x]]++;
      REP(x,9)
        if(cnt[b[y][x]] != 1)
          f[y][x] = false;
    }
    REP(x,9){
      int cnt[10] = {0};
      REP(y,9)
        cnt[b[y][x]]++;
      REP(y,9)
        if(cnt[b[y][x]] != 1)
          f[y][x] = false;
    }

    REP(i,9){
      REP(j,9)
        printf("%c%d",(f[i][j]?' ':'*'),b[i][j]);
      puts("");
    }
  }
  return 0;
}

C++: #include<iostream>
#include<string.h>
#include<vector>
#include<list>
#include<stdio.h>
#include<math.h>
#include<iomanip>
#include<map>
#include<stack>
#include<queue>
#define range(i,a,b) for(int i = (a); i < (b); i++)
#define rep(i,b) range(i,0,b)
#define debug(x) cout << "debug " << x << endl;
using namespace std;

int ma[9][9];

void solve(int y, int x, int check[][9]){
    rep(i, 9){
        if(ma[i][x] == ma[y][x]) check[i][x]++;
        if(ma[y][i] == ma[y][x]) check[y][i]++;
    }
    rep(i, 3){
        rep(j, 3){
            if(ma[i + y / 3 * 3][j + x / 3 * 3] == ma[y][x])
                check[i + y / 3 * 3][j + x / 3 * 3]++;
        }
    }
}

int main(){
    int n;
    while(cin >> n){
        rep(i, n){
            if(i) cout << endl;
            int check[9][9] = {0};
            rep(i, 9){
                rep(j, 9){
                    cin >> ma[i][j];
                }
            }
            rep(i, 9){
                rep(j, 9){
                    solve(i, j, check);
                }
            }
            rep(i,9){
                rep(j,9){
                    if(check[i][j] > 3)
                        cout << '*';
                    else
                        cout << ' ';
                    cout << ma[i][j];
                }
                cout << endl;
            }
        }
    }
}

C++: #include <iostream>
using namespace std;
int field[9][9];
char afield[9][19];

void search(int x,int y) {
  bool check=true;
  int temp = field[x][y];

  for(int i=0;i<9;i++) { //tate
    if(field[i][y] == temp && i != x){
      check = false;
    }
  }

  for(int i=0;i<9;i++) { //yoko
    if(field[x][i] == temp && i != y){
      check = false;
    }
  }

  for(int i=(x/3)*3;i<(x/3)*3+3;i++) { // 3*3 square
    for(int j=(y/3)*3;j<(y/3)*3+3;j++) {
      if((field[i][j] == temp) && (i != x) && (j != y)) {
	check = false;
      }
    }
  }

  if(check) {
    afield[x][y] = ' ';
  }
  else {
    afield[x][y] = '*';
  }
}

void solve() {
  for(int i=0;i<9;i++) {
    for(int j=0;j<9;j++) {
      search(i,j);
    }
  }
}

int main() {
  int n;
  string str;
  bool r=false;
  cin >> n;
  while(n--) {
    if(r) cout << endl;
    for(int i=0;i<9;i++) {
      for(int j=0;j<9;j++) {
	cin >> field[i][j];
      }
    }
    solve();
    for(int i=0;i<9;i++) {
      for(int j=0;j<9;j++) {
	cout << afield[i][j] << field[i][j];
      }
      cout << endl;
    }
    r = true;
  }
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <cstring>
using namespace std;

int puzzle[9][9];
bool ast[9][9];

void check() {
    for (int i=0; i<9; ++i) {
        for (int j=0; j<9; ++j) {
            for (int k=0; k<9; ++k) {
                if (i != k && puzzle[i][j] == puzzle[k][j]) {
                    ast[i][j] = true;
                    break;
                }
                if (j != k && puzzle[i][j] == puzzle[i][k]) {
                    ast[i][j] = true;
                    break;
                }
            }
            for (int k=(i/3)*3; k<(i/3)*3+3; ++k) {
                for (int l=(j/3)*3; l<(j/3)*3+3; ++l) {
                    if ((i != k || j != l) && puzzle[i][j] == puzzle[k][l]) {
                        ast[i][j] = true;
                        break;
                    }
                }
            }
        }
    }
}

int main() {
    int n;
    bool lf = false;
    cin >> n;
    for (int k=0; k<n; ++k) {
        if (lf) {
            cout << endl;
        }
        memset(ast, false, sizeof ast);
        for (int i=0; i<9; ++i) {
            for (int j=0; j<9; ++j) {
                cin >> puzzle[i][j];
            }
        }
        check();
        for (int i=0; i<9; ++i) {
            for (int j=0; j<9; ++j) {
                cout << (ast[i][j] ? "*" : " ") << puzzle[i][j];
            }
            cout << endl;
        }
        lf = true;
    }
    return 0;
}