Sudoku Solver Using Backtracking in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if a number is already in the given row.

DSA Javascript

function isInRow(board, row, num) {
  for (let col = 0; col < 9; col++) {
    if (board[row][col] === [1]) {
      return true;
    }
  }
  return false;
}

Drag options to blanks, or click blank then click option'

Anum

Brow

Cboard

Dcol

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong variable instead of the number to check.

We check if the current cell in the row equals the number we want to place.

2fill in blank

medium

Complete the code to check if a number is already in the given column.

DSA Javascript

function isInCol(board, col, num) {
  for (let row = 0; row < 9; row++) {
    if (board[row][col] === [1]) {
      return true;
    }
  }
  return false;
}

Drag options to blanks, or click blank then click option'

Acol

Brow

Cnum

Dboard

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong variable instead of the number to check.

We check if the current cell in the column equals the number we want to place.

3fill in blank

hard

Fix the error in checking if a number is in the 3x3 box.

DSA Javascript

function isInBox(board, startRow, startCol, num) {
  for (let row = 0; row < 3; row++) {
    for (let col = 0; col < 3; col++) {
      if (board[startRow + row][startCol + col] === [1]) {
        return true;
      }
    }
  }
  return false;
}

Drag options to blanks, or click blank then click option'

Anum

BstartRow

CstartCol

Drow

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing with wrong variables like startRow or startCol.

We compare each cell in the 3x3 box with the number to check if it is already present.

4fill in blank

hard

Fill both blanks to check if placing a number is valid in the Sudoku board.

DSA Javascript

function isValid(board, row, col, num) {
  return !isInRow(board, row, num) && !isInCol(board, col, num) && !isInBox(board, row - row % 3, col - col % 3, [1]) && board[row][col] === [2];
}

Drag options to blanks, or click blank then click option'

Anum

B0

Cnull

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Using null or undefined instead of 0 for empty cells.

We check the number for the box and ensure the cell is empty (0) before placing.

5fill in blank

hard

Fill all three blanks to complete the backtracking solver function.

DSA Javascript

function solveSudoku(board) {
  for (let row = 0; row < 9; row++) {
    for (let col = 0; col < 9; col++) {
      if (board[row][col] === [1]) {
        for (let num = 1; num <= 9; num++) {
          if (isValid(board, row, col, num)) {
            board[row][col] = [2];
            if (solveSudoku(board)) {
              return true;
            } else {
              board[row][col] = [3];
            }
          }
        }
        return false;
      }
    }
  }
  return true;
}

Drag options to blanks, or click blank then click option'

A0

Bnum

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Using null instead of 0 for empty cells.

Not resetting the cell on backtrack.

We find empty cells (0), try numbers (num), place them, and backtrack by resetting to 0 if needed.

Practice

1. What is the main idea behind using backtracking to solve a Sudoku puzzle?

easy

A. Try placing numbers one by one and backtrack if a conflict occurs

B. Fill all empty cells randomly and check if the puzzle is solved

C. Use a sorting algorithm to arrange numbers in order

D. Replace all zeros with the number 9 directly

Solution

1. Step 1: Understand backtracking approach

   Backtracking tries numbers in empty cells one by one and checks if they fit the Sudoku rules.

2. Step 2: Handle conflicts by backtracking

   If a number causes a conflict, the algorithm removes it and tries the next number, going back if needed.

3. Final Answer:

   Try placing numbers one by one and backtrack if a conflict occurs -> Option A

4. Quick Check:

   Backtracking = Try and undo [OK]

Hint: Backtracking means try, check, undo if wrong [OK]

Common Mistakes:

• Thinking backtracking fills cells randomly
• Assuming sorting solves Sudoku
• Replacing zeros without checks

2. Which of the following is the correct way to check if a number can be placed in a Sudoku cell in JavaScript?

easy

A. if isSafe(board, row, col, num) { placeNumber(); }

B. if isSafe(board, row, col, num) then placeNumber();

C. if (isSafe board, row, col, num) { placeNumber(); }

D. if (isSafe(board, row, col, num)) { placeNumber(); }

Solution

1. Step 1: Understand JavaScript if syntax

   JavaScript requires parentheses around the condition in an if statement.

2. Step 2: Check function call syntax

   Function calls need parentheses with arguments inside, like isSafe(board, row, col, num).

3. Final Answer:

   if (isSafe(board, row, col, num)) { placeNumber(); } -> Option D

4. Quick Check:

   if(condition) with parentheses [OK]

Hint: Use parentheses around conditions in if statements [OK]

Common Mistakes:

• Missing parentheses in if condition
• Using 'then' keyword in JavaScript
• Incorrect function call syntax

3. What will be the output of the following code snippet when solving a Sudoku board?

const board = [
  [5,3,0,0,7,0,0,0,0],
  [6,0,0,1,9,5,0,0,0],
  [0,9,8,0,0,0,0,6,0],
  [8,0,0,0,6,0,0,0,3],
  [4,0,0,8,0,3,0,0,1],
  [7,0,0,0,2,0,0,0,6],
  [0,6,0,0,0,0,2,8,0],
  [0,0,0,4,1,9,0,0,5],
  [0,0,0,0,8,0,0,7,9]
];
solveSudoku(board);
console.log(board);

medium

A. An error because solveSudoku is not defined

B. The original board unchanged with zeros intact

C. A fully solved Sudoku board with no zeros

D. A partially solved board with some zeros replaced

Solution

1. Step 1: Understand solveSudoku function purpose

   solveSudoku uses backtracking to fill all empty cells correctly, solving the puzzle.

2. Step 2: Check console.log output after solving

   After solveSudoku runs, the board is fully solved with no zeros remaining.

3. Final Answer:

   A fully solved Sudoku board with no zeros -> Option C

4. Quick Check:

   Backtracking solves all cells [OK]

Hint: Backtracking fills all empty cells correctly [OK]

Common Mistakes:

• Expecting original board unchanged
• Thinking partial fill is final
• Assuming function is undefined

4. Consider this buggy code snippet for checking if a number can be placed in a Sudoku cell:

function isSafe(board, row, col, num) {
  for (let x = 0; x < 9; x++) {
    if (board[row][x] === num || board[x][col] === num) {
      return false;
    }
  }
  // Missing 3x3 box check
  return true;
}

What is the main problem here?

medium

A. It returns false too early

B. It does not check the 3x3 subgrid for duplicates

C. It uses wrong loop variable x

D. It modifies the board inside the function

Solution

1. Step 1: Analyze row and column checks

   The code correctly checks if num exists in the same row or column.

2. Step 2: Identify missing 3x3 subgrid check

   Sudoku rules require checking the 3x3 box; this code does not do that, causing errors.

3. Final Answer:

   It does not check the 3x3 subgrid for duplicates -> Option B

4. Quick Check:

   3x3 box check missing = bug [OK]

Hint: Always check row, column, and 3x3 box [OK]

Common Mistakes:

• Ignoring 3x3 box validation
• Thinking loop variable causes error
• Assuming early return is wrong

5. You want to optimize the Sudoku solver by choosing the next empty cell with the fewest possible valid numbers to try. Which technique does this describe?

hard

A. Heuristic with Minimum Remaining Value (MRV) strategy

B. Random guessing of empty cells

C. Depth-first search without pruning

D. Sorting the board rows before solving

Solution

1. Step 1: Understand MRV heuristic

   MRV chooses the empty cell with the fewest valid options to reduce branching and speed up solving.

2. Step 2: Compare with other options

   Random guessing and depth-first search without pruning are less efficient; sorting rows does not help solving.

3. Final Answer:

   Heuristic with Minimum Remaining Value (MRV) strategy -> Option A

4. Quick Check:

   MRV = choose cell with fewest options [OK]

Hint: Pick cell with fewest choices first [OK]

Common Mistakes:

• Confusing random guessing with heuristics
• Ignoring pruning benefits
• Thinking sorting helps solving