Word Search in Grid Using Backtracking in DSA C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main idea behind using backtracking for word search in a grid?

Backtracking tries all possible paths in the grid to find the word by moving step-by-step and undoing moves if they don't lead to a solution.

beginner

In a word search grid, how do we ensure we don't use the same cell twice in one word path?

We mark the cell as visited during the current path and unmark it when backtracking to allow other paths to use it.

beginner

What directions are typically checked when searching for the next character in the grid?

We check up, down, left, and right neighbors of the current cell to find the next character.

intermediate

Why is backtracking suitable for word search problems in grids?

Because it explores all possible paths systematically and stops when the word is found or all options are exhausted.

intermediate

What is the base case in the recursive backtracking function for word search?

The base case is when all characters of the word have been matched successfully, meaning the word is found.

Which of the following is NOT a valid move when searching for the next character in the grid?

A Moving up

B Moving diagonally

C Moving left

D Moving right

Typically, only up, down, left, and right moves are allowed, not diagonal moves.

What should you do if the current cell's character does not match the word's current character?

A Backtrack and try a different path

B Return true immediately

C Mark the cell as visited

D Continue searching neighbors

If the character does not match, backtracking means undoing the current step and trying other paths.

How do you prevent revisiting the same cell in the current search path?

A Mark cells as visited temporarily during the current path

B Ignore visited cells permanently

C Use a global visited array without resetting

D Do not check neighbors

Cells are marked visited only during the current path and unmarked when backtracking.

What does the backtracking function return when the entire word is found?

A The length of the word

B False

C True

D The current position

It returns true to indicate the word has been found.

Which data structure is commonly used to keep track of visited cells in the grid?

A Stack

B Queue

C Linked list

D 2D boolean array

A 2D boolean array matching the grid size is used to mark visited cells.

Explain how backtracking works to find a word in a grid step-by-step.

Think about trying paths and undoing moves if they don't work.

Describe how you would implement a function to check if a word exists in a grid using backtracking.

Focus on recursion and visited tracking.

Practice

1. What is the main purpose of using backtracking in a word search grid problem?

easy

A. To sort the grid alphabetically before searching

B. To explore all possible paths to find the word in the grid

C. To count the number of words in the grid

D. To replace characters in the grid with a placeholder

Solution

1. Step 1: Understand backtracking in grid search

   Backtracking tries all possible paths from a starting point to find the target word.

2. Step 2: Compare options with backtracking purpose

   Only To explore all possible paths to find the word in the grid describes exploring all paths, which matches backtracking's goal.

3. Final Answer:

   To explore all possible paths to find the word in the grid -> Option B

4. Quick Check:

   Backtracking = explore all paths [OK]

Hint: Backtracking means trying all paths step-by-step [OK]

Common Mistakes:

• Thinking backtracking sorts or counts words
• Confusing backtracking with simple search
• Assuming backtracking modifies the grid permanently

2. Which of the following is the correct way to check if a cell (row, col) is inside the grid boundaries in C?

easy

A. if (row >= 1 && row < rows && col >= 1 && col < cols)

B. if (row > 0 && row <= rows && col > 0 && col <= cols)

C. if (row > 0 && row < rows && col >= 0 && col <= cols)

D. if (row >= 0 && row < rows && col >= 0 && col < cols)

Solution

1. Step 1: Understand zero-based indexing in C arrays

   Arrays start at index 0, so valid indices are from 0 to rows-1 and 0 to cols-1.

2. Step 2: Check each option for correct boundary conditions

   if (row >= 0 && row < rows && col >= 0 && col < cols) correctly uses >= 0 and < rows/cols. Others use > 0 or <= rows which are incorrect for zero-based arrays.

3. Final Answer:

   if (row >= 0 && row < rows && col >= 0 && col < cols) -> Option D

4. Quick Check:

   Check boundaries with >=0 and < size [OK]

Hint: Use zero-based index: start at 0, less than size [OK]

Common Mistakes:

• Using > 0 instead of >= 0 excludes first row/column
• Using <= rows includes out-of-bound index
• Mixing 1-based and 0-based indexing

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

char grid[3][3] = {{'A','B','C'},{'D','E','F'},{'G','H','I'}};
int found = searchWord(grid, 3, 3, "BEF");
printf("%d", found);

Assuming searchWord correctly implements backtracking to find the word in the grid.

medium

A. 1

B. 0

C. Compilation error

D. Runtime error

Solution

1. Step 1: Analyze the grid and the word "BEF"

   The grid rows are: ABC, DEF, GHI. The word "BEF" requires letters B, E, F in sequence adjacent horizontally or vertically.

2. Step 2: Check if "BEF" exists in adjacent cells

   B is at (0,1), E is at (1,1), F is at (1,2). B to E is vertical down, E to F is horizontal right, so path exists.

3. Step 3: Confirm if backtracking finds the word

   Backtracking can follow B(0,1) -> E(1,1) -> F(1,2), so word is found.

4. Final Answer:

   1 -> Option A

5. Quick Check:

   Word found = 1 [OK]

Hint: Trace letters stepwise in grid adjacency [OK]

Common Mistakes:

• Assuming letters must be in same row only
• Ignoring vertical or diagonal adjacency rules
• Confusing output 0 and 1 meaning

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

bool dfs(char grid[][], int row, int col, int index, char* word) {
  if (index == strlen(word)) return true;
  if (row < 0 || row >= rows || col < 0 || col >= cols) return false;
  if (grid[row][col] != word[index]) return false;
  char temp = grid[row][col];
  grid[row][col] = '#';
  bool found = dfs(grid, row+1, col, index+1, word) ||
               dfs(grid, row-1, col, index+1, word) ||
               dfs(grid, row, col+1, index+1, word) ||
               dfs(grid, row, col-1, index+1, word);
  grid[row][col] = temp;
  return found;
}

medium

A. The base case should check index > strlen(word) instead of ==

B. The grid cell is not restored after recursion

C. The grid parameter lacks column size, causing compilation error

D. The function does not check diagonal neighbors

Solution

1. Step 1: Check function parameter declaration

   In C, 2D arrays must have fixed column size in parameters, e.g., char grid[][cols]. Missing column size causes compilation error.

2. Step 2: Verify other parts of the code

   Base case with index == strlen(word) is correct. Grid cell is restored after recursion. Diagonal check is optional, not a bug.

3. Final Answer:

   The grid parameter lacks column size, causing compilation error -> Option C

4. Quick Check:

   2D array params need column size [OK]

Hint: Always specify column size in 2D array parameters [OK]

Common Mistakes:

• Ignoring 2D array parameter syntax in C
• Thinking base case condition is wrong
• Assuming diagonal moves are mandatory

5. Given a 4x4 grid:

{ {'C','A','T','F'},
  {'B','G','E','S'},
  {'I','T','A','E'},
  {'S','O','N','G'} }

Which of the following words can be found using backtracking with adjacent horizontal or vertical moves only?

hard

A. "SONG"

B. "BATS"

C. "CATS"

D. "FATE"

Solution

1. Step 1: Check each word's presence in the grid with adjacency rules

   "SONG": S(3,0), O(3,1), N(3,2), G(3,3) - all adjacent horizontally, so possible.
   "BATS": B(1,0) has no adjacent A (neighbors: C(0,0), I(2,0), G(1,1)), so no.
   "CATS": C(0,0), A(0,1), T(0,2) adjacent, but T(0,2) has no adjacent S (neighbors: A(0,1), F(0,3), E(1,2), A(2,2)), so no.
   "FATE": F(0,3) has no adjacent A (neighbors: T(0,2), S(1,3)), so no.

2. Step 2: Identify which word is fully connected by adjacent moves

   Only "SONG" has all letters adjacent in sequence.

3. Final Answer:

   "SONG" -> Option A

4. Quick Check:

   Check adjacency path for each word [OK]

Hint: Trace letters stepwise horizontally or vertically [OK]

Common Mistakes:

• Assuming diagonal moves allowed
• Ignoring adjacency between letters
• Choosing words with missing adjacent letters