Sudoku Solver Using Backtracking in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

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

DSA C

int usedInRow(int grid[9][9], int row, int num) {
    for (int col = 0; col < 9; col++) {
        if (grid[row][col] == [1]) {
            return 1;
        }
    }
    return 0;
}

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

Anum

Brow

Ccol

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing with 'row' or 'col' instead of 'num'.

Using assignment '=' instead of comparison '=='.

We check if the number 'num' is present in the given row by comparing each cell in that row with 'num'.

2fill in blank

medium

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

DSA C

int usedInCol(int grid[9][9], int col, int num) {
    for (int row = 0; row < 9; row++) {
        if (grid[row][col] == [1]) {
            return 1;
        }
    }
    return 0;
}

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

Anum

Bcol

Crow

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing with 'row' or 'col' instead of 'num'.

Using assignment '=' instead of comparison '=='.

We check if 'num' is present in the given column by comparing each cell in that column with 'num'.

3fill in blank

hard

Fix the error in the code that checks if a number is used in the 3x3 box.

DSA C

int usedInBox(int grid[9][9], int boxStartRow, int boxStartCol, int num) {
    for (int row = 0; row < 3; row++) {
        for (int col = 0; col < 3; col++) {
            if (grid[row + boxStartRow][col + boxStartCol] == [1]) {
                return 1;
            }
        }
    }
    return 0;
}

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AboxStartCol

Bnum

CboxStartRow

Drow

Attempts:

3 left

💡 Hint

Common Mistakes

Comparing with boxStartRow or boxStartCol instead of 'num'.

Using assignment '=' instead of comparison '=='.

We must check if the cell value equals 'num' to detect if 'num' is already in the 3x3 box.

4fill in blank

hard

Fill both blanks to complete the function that checks if it is safe to place a number in a cell.

DSA C

int isSafe(int grid[9][9], int row, int col, int num) {
    return !usedInRow(grid, row, num) && !usedInCol(grid, col, num) && !usedInBox(grid, row - row % 3, col - col % 3, [1]) && grid[row][col] == [2];
}

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

Anum

B0

Crow

Dcol

Attempts:

3 left

💡 Hint

Common Mistakes

Using row or col instead of num in usedInBox.

Checking if grid[row][col] equals num instead of 0.

We check if 'num' is not in row, column, or box, and the cell is empty (0) before placing 'num'.

5fill in blank

hard

Fill all three blanks to complete the backtracking function that solves the Sudoku.

DSA C

int solveSudoku(int grid[9][9]) {
    int row, col;
    int found = 0;
    for (row = 0; row < 9; row++) {
        for (col = 0; col < 9; col++) {
            if (grid[row][col] == 0) {
                found = 1;
                break;
            }
        }
        if (found) break;
    }
    if (!found) return 1;

    for (int num = 1; num <= 9; num++) {
        if (isSafe(grid, row, col, [1])) {
            grid[row][col] = [2];
            if (solveSudoku(grid))
                return [3];
            grid[row][col] = 0;
        }
    }
    return 0;
}

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

Anum

Brow

C1

Dcol

Attempts:

3 left

💡 Hint

Common Mistakes

Using row or col instead of num for placement.

Returning 0 instead of 1 on success.

We try numbers 1 to 9 (num) in the empty cell. If safe, place num and recurse. If solved, return 1 (true).