Word Search in Grid Using Backtracking in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to check if the current cell matches the current character of the word.

DSA C#

if (board[row, col] == word[[1]])
{
    // proceed
}

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

Aword.Length

Brow

Ccol

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using row or col instead of the word index.

Using word.Length which is out of range.

We compare the board cell with the character at the current index in the word.

2fill in blank

medium

Complete the code to mark the current cell as visited.

DSA C#

visited[row, col] = [1];

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

Atrue

B0

Cfalse

Dnull

Attempts:

3 left

💡 Hint

Common Mistakes

Marking visited as false instead of true.

Using null or 0 which are invalid for boolean arrays.

We mark the cell as visited by setting it to true.

3fill in blank

hard

Fix the error in the recursive call to explore neighbors.

DSA C#

bool found = DFS(board, word, row + [1], col, index + 1, visited) ||
             DFS(board, word, row - 1, col, index + 1, visited);

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

A1

B-1

C0

Dindex

Attempts:

3 left

💡 Hint

Common Mistakes

Using -1 to move down which actually moves up.

Using 0 which does not move.

We move down by adding 1 to the row index.

4fill in blank

hard

Fill both blanks to check boundaries and visited status before recursion.

DSA C#

if (row >= 0 && row < board.GetLength(0) && col >= 0 && col < board.GetLength(1) && !visited[[1], [2]])
{
    // safe to explore
}

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

Arow

Bcol

Cindex

Dword

Attempts:

3 left

💡 Hint

Common Mistakes

Using index or word instead of row and col for visited check.

Mixing row and col order.

We check if the cell at (row, col) is not visited before exploring.

5fill in blank

hard

Fill all three blanks to backtrack correctly after exploring neighbors.

DSA C#

visited[[1], [2]] = [3];

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

Arow

Bcol

Cfalse

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Setting visited to true again instead of false.

Using wrong indices for visited.

After exploring, mark the cell as not visited (false) to allow other paths.

Practice

1. What is the main purpose of backtracking in the Word Search problem on a grid?

easy

A. To count the number of words in the grid

B. To sort the grid alphabetically

C. To explore all possible paths to find the word

D. To replace characters in the grid

Solution

1. Step 1: Understand backtracking role

   Backtracking tries all possible paths in the grid to find the target word by moving in allowed directions.

2. Step 2: Eliminate other options

   Sorting, counting, or replacing characters are unrelated to the search process using backtracking.

3. Final Answer:

   To explore all possible paths to find the word -> Option C

4. Quick Check:

   Backtracking = Explore paths [OK]

Hint: Backtracking explores paths step-by-step to find the word [OK]

Common Mistakes:

• Thinking backtracking sorts or modifies the grid
• Confusing backtracking with counting words
• Assuming backtracking replaces characters permanently

2. Which of the following is the correct way to check if the next cell is within grid bounds in C#?

easy

A. if (row >= 0 && row < grid.Length && col >= 0 && col < grid[0].Length)

B. if (row > 0 && row <= grid.Length && col > 0 && col <= grid[0].Length)

C. if (row >= 0 && row <= grid.Length && col >= 0 && col <= grid[0].Length)

D. if (row > 0 && row < grid.Length && col > 0 && col < grid[0].Length)

Solution

1. Step 1: Understand grid indexing in C#

   Grid rows and columns are zero-indexed, so valid indices are from 0 to Length-1.

2. Step 2: Check boundary conditions

   if (row >= 0 && row < grid.Length && col >= 0 && col < grid[0].Length) correctly checks if row and col are >= 0 and less than Length, ensuring valid indices.

3. Final Answer:

   if (row >= 0 && row < grid.Length && col >= 0 && col < grid[0].Length) -> Option A

4. Quick Check:

   Bounds check uses >=0 and < Length [OK]

Hint: Use >=0 and < Length for zero-based index checks [OK]

Common Mistakes:

• Using > 0 excludes first row/column
• Using <= Length causes out-of-range errors
• Confusing row and column length checks

3. What will be the output of the following C# code snippet for word search?

char[][] grid = {
  new char[] {'A','B','C','E'},
  new char[] {'S','F','C','S'},
  new char[] {'A','D','E','E'}
};
string word = "ABCCED";
bool result = WordSearch(grid, word);
Console.WriteLine(result);

Assuming WordSearch uses backtracking correctly to find the word in the grid.

medium

A. True

B. False

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Analyze the grid and word

   The word "ABCCED" can be found by moving right and down starting at grid[0][0]: A->B->C->C->E->D.

2. Step 2: Confirm backtracking finds the word

   Backtracking explores all paths and will find this sequence, so result is true.

3. Final Answer:

   True -> Option A

4. Quick Check:

   Word found in grid = True [OK]

Hint: Trace word path step-by-step in grid to confirm presence [OK]

Common Mistakes:

• Assuming word is not found without tracing path
• Confusing similar letters in grid
• Ignoring backtracking's ability to explore all paths

4. Identify the bug in this backtracking snippet for word search:

bool Backtrack(char[][] grid, string word, int index, int row, int col) {
  if (index == word.Length) return true;
  if (row < 0 || row >= grid.Length || col < 0 || col >= grid[0].Length) return false;
  if (grid[row][col] != word[index]) return false;
  char temp = grid[row][col];
  grid[row][col] = '#';
  bool found = Backtrack(grid, word, index + 1, row + 1, col) ||
               Backtrack(grid, word, index + 1, row - 1, col) ||
               Backtrack(grid, word, index + 1, row, col + 1) ||
               Backtrack(grid, word, index + 1, row, col - 1);
  grid[row][col] = temp;
  return found;
}

medium

A. The grid cell is not restored after recursion

B. The function does not check for visited cells properly

C. The base case should check index > word.Length

D. The function does not check grid boundaries before accessing

Solution

1. Step 1: Check visited cell marking

   The code marks visited cells by replacing with '#', which prevents revisiting during recursion.

2. Step 2: Identify missing visited check

   However, the code does not check if the current cell is '#' before comparing with word[index], so it may revisit marked cells incorrectly.

3. Final Answer:

   The function does not check for visited cells properly -> Option B

4. Quick Check:

   Visited check missing = Bug [OK]

Hint: Always check if cell is visited before matching character [OK]

Common Mistakes:

• Forgetting to check if cell is marked visited
• Incorrect base case condition
• Not restoring grid cell after recursion

5. Given a 4x4 grid:

char[][] grid = {
  new char[] {'C','A','T','F'},
  new char[] {'B','G','E','S'},
  new char[] {'I','T','A','E'},
  new char[] {'S','O','N','G'}
};

Which of the following words cannot be found using backtracking moving only up, down, left, or right?

hard

A. "CAT"

B. "GATE"

C. "SONG"

D. "BATS"

Solution

1. Step 1: Check presence of each word

   "CAT" is in first row (C->A->T). "SONG" is in last row (S->O->N->G). "GATE" can be formed by G->A->T->E in middle rows.

2. Step 2: Verify "BATS" formation

   "BATS" requires B->A->T->S. B is at grid[1][0], but no path connects A and T and then S in sequence moving only up/down/left/right without skipping letters.

3. Final Answer:

   "BATS" cannot be found -> Option D

4. Quick Check:

   Word path missing = "BATS" [OK]

Hint: Trace each letter path carefully to confirm word presence [OK]

Common Mistakes:

• Assuming all words are present without path check
• Ignoring movement restrictions (no diagonals)
• Confusing letter positions in grid