SMMR-142: Sudoku [Extra Challenge]

#include <bits/stdc++.h>
using namespace std;
#define N 10
using pii=pair<int,int>;
int ar[N][N];

bool fill_line(int line,int num){
    for(int j=1;j<=9;j++)
        if(ar[line][j]==num) return false;
    return true;
}
bool fill_col(int col,int num){
    for(int i=1;i<=9;i++)
        if(ar[i][col]==num) return false;
    return true;
}
bool fill_square(int line,int col,int num){
    if(1<=line&&line<=3) line=1;
    else if(4<=line&&line<=6) line=4;
    else line=7;
    if(1<=col&&col<=3) col=1;
    else if(4<=col&&col<=6) col=4;
    else col=7;
    for(int i=line;i<=line+2;i++){
        for(int j=col;j<=col+2;j++){
            if(ar[i][j]==num) return false;
        }
    }
    return true;
}

pii find_zero(int ar[N][N]){
    for(int i=1;i<=9;i++){
        for(int j=1;j<=9;j++){
            if(ar[i][j]==0)
            return {i,j};
        }
    }
    return {-1,-1};
}

bool sudoku(int ar[N][N]){

    int line,col;
    line=find_zero(ar).first;
    col=find_zero(ar).second;
    if(line==-1&&col==-1) return true;

    for(int num=1;num<=9;num++){
        if( fill_line(line,num) && fill_col(col,num) && fill_square(line,col,num) ){
            ar[line][col]=num;
            if( sudoku(ar) ) return true;
            ar[line][col]=0;
        }
    }

    return false;
}

int main(){

    FILE *read,*write;
    read=fopen("input_sudoku.txt","r");
    write=fopen("sudoku_ans.txt","w");
    string  str[10];
    int t=0;
    while(fscanf(read,"%[^\n]s",str)!=EOF){

        for(int i=1;i<=9;i++){
            for(int j=1;j<=9;j++){
                fscanf(read,"%d",&ar[i][j]);
            }
        }

        sudoku(ar);
        fprintf(write,"%s\n",str);
        for(int i=1;i<=9;i++){
            for(int j=1;j<=9;j++){
                fprintf(write,"%d ",ar[i][j]);
            }
            fprintf(write,"\n");
        }

        fscanf(read,"%[^\n]s",str);
        for(int i=1;i<=9;i++){
            for(int j=1;j<=9;j++){
                fscanf(read,"%d",&ar[i][j]);
            }
        }
    }
    fclose(read);
    fclose(write);
    return 0;
}