script.js

var arr = [[], [], [], [], [], [], [], [], []]

for (var i = 0; i < 9; i++) {
	for (var j = 0; j < 9; j++) {
		arr[i][j] = document.getElementById(i * 9 + j);

	}
}


var board = [[], [], [], [], [], [], [], [], []]

function FillBoard(board) {
	for (var i = 0; i < 9; i++) {
		for (var j = 0; j < 9; j++) {
			if (board[i][j] != 0) {
				arr[i][j].innerText = board[i][j]
			}

			else
				arr[i][j].innerText = ''
		}
	}
}

let GetPuzzle = document.getElementById('GetPuzzle')
let SolvePuzzle = document.getElementById('SolvePuzzle')

GetPuzzle.onclick = function () {
	var xhrRequest = new XMLHttpRequest()
	xhrRequest.onload = function () {
		var response = JSON.parse(xhrRequest.response)
		console.log(response)
		board = response.board
		FillBoard(board)
	}
	xhrRequest.open('get', 'https://sugoku.onrender.com/board?difficulty=easy')
	//we can change the difficulty of the puzzle the allowed values of difficulty are easy, medium, hard and random
	xhrRequest.send()
}

SolvePuzzle.onclick = () => {
	SudokuSolver(board, 0, 0, 9);
};

function isValid(board ,i,j, num,n){
    //row and column check
   for(let x=0;x<n;x++){
       if(board[i][x]==num || board[x][j]==num){
           return false;
       }
   }
   //submatrix check
   let rn=Math.sqrt(n);
   let si=i-i%rn;
   let sj=j-j%rn;
    
    for(let x=si;x<si;x++){
        for(let y=sj;y<sj+rn;y++){
            if(board[x][y]==num){
                return false;
            }
        }
    }
    return true;
}

function SudokuSolver(board,
i,j,n){
    //Base case
    if(i==n){
        //Print(board,n);
		FillBoard(board)
        return true;
    }
    //if we are not inside the board
    if(j==n){
        return SudokuSolver(board,i+1,0,n);
    }
    //if cell is already filled -> just move ahead
    if(board[i][j]!=0){
        return SudokuSolver(board,i,j+1,n);
    }
    
    //we try to fill the cell with an appropriate number
    for(let num=1;num<=9;num++){
        //Check if the number can be filled
        if(isValid(board,i,j,num,n)){
            board[i][j]=num;
            let subAns = SudokuSolver(board,i,j+1,n);
            if(subAns){
                return true;
            }
            //Backtracking ->undo the change
            board[i][j]=0;
            
        }
    }
    return false;
    
}