Backtracking Concept and Decision Tree Visualization in DSA C - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start the backtracking recursion from the first position.

DSA C

void solve() {
    int start = [1];
    backtrack(start);
}

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

ANULL

B1

C-1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Starting from 1 instead of 0

Using NULL for an integer index

The backtracking usually starts from index 0 to cover all possibilities from the beginning.

2fill in blank

medium

Complete the code to check if the current choice is valid before proceeding.

DSA C

if (isValid([1])) {
    backtrack(pos + 1);
}

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

A0

Bpos + 1

Cpos

DNULL

Attempts:

3 left

💡 Hint

Common Mistakes

Passing pos + 1 instead of pos

Passing NULL or 0 incorrectly

We check if the current position choice is valid before moving forward.

3fill in blank

hard

Fix the error in the backtracking base case to stop recursion correctly.

DSA C

if (pos == [1]) {
    printSolution();
    return;
}

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

An

B0

C-1

Dpos

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 or -1 as base case

Using pos instead of n

The base case is when the position reaches n, meaning all positions are processed.

4fill in blank

hard

Fill both blanks to correctly undo the choice after backtracking.

DSA C

choices[pos] = [1];
backtrack(pos + 1);
choices[pos] = [2];

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

Atrue

Bfalse

C0

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Not undoing the choice

Using 0 and -1 inconsistently

We mark the choice as made (true), recurse, then undo it (false) to explore other options.

5fill in blank

hard

Fill all three blanks to build a decision tree node and recurse correctly.

DSA C

struct Node {
    int choice;
    struct Node* [1];
    struct Node* [2];
};

void explore(struct Node* node) {
    if (node == NULL) return;
    process(node->choice);
    explore(node->[3]);
}

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

Aleft

Bright

Cchild

Dnext

Attempts:

3 left

💡 Hint

Common Mistakes

Using child or next instead of left/right

Recursing on right instead of left

The decision tree node has left and right children. We recurse on the left child in explore.

Practice

1. What is the main idea behind the backtracking technique in algorithms?

easy

A. Divide the problem into smaller independent parts

B. Sort the data before processing

C. Try all possible options and undo choices when they lead to no solution

D. Use a queue to process elements in order

Solution

1. Step 1: Understand backtracking approach

   Backtracking tries all possible options step-by-step and undoes choices if they don't lead to a solution.

2. Step 2: Compare with other options

   Sorting, queues, and dividing problems are different techniques, not backtracking.

3. Final Answer:

   Try all possible options and undo choices when they lead to no solution -> Option C

4. Quick Check:

   Backtracking = Try and undo choices [OK]

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

Common Mistakes:

• Confusing backtracking with sorting or divide and conquer
• Thinking backtracking only tries one path
• Ignoring the undo step in backtracking

2. Which of the following is the correct syntax to start a backtracking function in C that takes an integer step as input?

easy

A. void backtrack(int step) { }

B. int backtrack() { step; }

C. backtrack(int step) void { }

D. void backtrack step(int) { }

Solution

1. Step 1: Recall C function syntax

   In C, a function with no return value uses void, name, and parameters in parentheses.

2. Step 2: Check each option

   void backtrack(int step) { } matches correct syntax: void backtrack(int step) { }. Others have wrong order or missing parts.

3. Final Answer:

   void backtrack(int step) { } -> Option A

4. Quick Check:

   Correct C function syntax = void backtrack(int step) { } [OK]

Hint: C functions: return_type name(params) { } [OK]

Common Mistakes:

• Placing parameter type after name incorrectly
• Missing parentheses around parameters
• Wrong order of return type and function name

3. Consider the following C code snippet for backtracking to generate binary strings of length 2:

void backtrack(int step, char *arr) {
  if (step == 2) {
    printf("%s\n", arr);
    return;
  }
  arr[step] = '0';
  backtrack(step + 1, arr);
  arr[step] = '1';
  backtrack(step + 1, arr);
}

int main() {
  char arr[3] = {0};
  backtrack(0, arr);
  return 0;
}

What is the output of this program?

medium

A. 0
1
0
1

B. 00
01
10
11

C. 00
11

D. Syntax error

Solution

1. Step 1: Understand recursion and base case

   When step == 2, the string is printed. The array holds characters '0' or '1' at each step.

2. Step 2: Trace recursive calls

   At step 0: arr[0] = '0', recurse; then arr[0] = '1', recurse. At step 1: arr[1] = '0', recurse; then arr[1] = '1', recurse. This generates all 2-length binary strings.

3. Final Answer:

   00 01 10 11 -> Option B

4. Quick Check:

   All 2-length binary strings printed [OK]

Hint: Backtracking prints all combos by changing one position at a time [OK]

Common Mistakes:

• Not setting null terminator for string
• Printing before base case reached
• Confusing step count with array length

4. The following backtracking code is intended to generate all permutations of the array {1, 2}. What is the error?

void backtrack(int *arr, int start, int end) {
  if (start == end) {
    for (int i = 0; i <= end; i++) printf("%d", arr[i]);
    printf("\n");
    return;
  }
  for (int i = start; i <= end; i++) {
    int temp = arr[start];
    arr[start] = arr[i];
    arr[i] = temp;
    backtrack(arr, start + 1, end);
    // Missing swap back here
  }
}

int main() {
  int arr[] = {1, 2};
  backtrack(arr, 0, 1);
  return 0;
}

medium

A. The base case condition is incorrect

B. The array is not initialized properly

C. The loop should run from 0 to end, not start to end

D. The code misses swapping back after recursion, causing wrong output

Solution

1. Step 1: Analyze swapping logic

   The code swaps elements before recursion but does not swap them back after recursion, so the array stays changed incorrectly.

2. Step 2: Understand effect on output

   Without swapping back, permutations are not generated correctly because the original order is lost.

3. Final Answer:

   The code misses swapping back after recursion, causing wrong output -> Option D

4. Quick Check:

   Swap back needed to restore array [OK]

Hint: Always undo swaps after recursive calls in permutations [OK]

Common Mistakes:

• Forgetting to swap back after recursion
• Wrong loop range for permutations
• Incorrect base case check

5. You want to use backtracking to solve the N-Queens problem for N=4. Which of the following best describes the decision tree visualization for this problem?

hard

A. A tree where each level represents placing a queen in a row, branching by valid columns

B. A tree where each node represents a queen placed anywhere on the board without restrictions

C. A linear chain of nodes representing queens placed one after another without backtracking

D. A tree where each level represents removing queens from the board

Solution

1. Step 1: Understand N-Queens backtracking

   Backtracking places queens row by row, trying valid columns only to avoid attacks.

2. Step 2: Visualize decision tree

   Each tree level corresponds to a row; branches represent valid column choices for that queen.

3. Final Answer:

   A tree where each level represents placing a queen in a row, branching by valid columns -> Option A

4. Quick Check:

   Decision tree levels = rows, branches = valid columns [OK]

Hint: Decision tree levels = rows, branches = valid queen positions [OK]

Common Mistakes:

• Ignoring constraints when placing queens
• Thinking decision tree is linear without branches
• Confusing placement with removal steps