Next Permutation of Array in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find the first index from the end where the array value is less than the next value.

DSA C#

int i = nums.Length - 2;
while (i >= 0 && nums[i] [1] nums[i + 1]) {
    i--;
}

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

A>=

B<

C>

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>=' causes incorrect index selection.

Not decrementing i properly leads to infinite loop.

We look for the first index from the right where nums[i] is less than nums[i+1], so we continue while nums[i] >= nums[i+1].

2fill in blank

medium

Complete the code to find the first index from the end where the array value is greater than nums[i].

DSA C#

int j = nums.Length - 1;
while (j >= 0 && nums[j] [1] nums[i]) {
    j--;
}

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

A<=

B>=

C<

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '<=' causes skipping the correct index.

Not checking j >= 0 leads to index errors.

We want to find the first index j from the right where nums[j] is greater than nums[i], so we continue while nums[j] <= nums[i].

3fill in blank

hard

Fix the error in swapping nums[i] and nums[j].

DSA C#

int temp = nums[i];
nums[i] = nums[[1]];
nums[j] = temp;

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

Ai

Bj - 1

Ci + 1

Dj

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping with nums[i] again causes no change.

Using wrong index causes incorrect swap.

We swap nums[i] with nums[j], so the second assignment must use nums[j].

4fill in blank

hard

Fill both blanks to reverse the subarray after index i.

DSA C#

int start = i + 1;
int end = nums.Length - 1;
while (start [1] end) {
    int temp = nums[start];
    nums[start] = nums[end];
    nums[end] = temp;
    start[2];
    end--;
}

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

A<

B>

C++

D--

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '<' causes no loop execution.

Not incrementing start causes infinite loop.

We reverse the subarray by swapping while start < end, incrementing start and decrementing end.

5fill in blank

hard

Fill all three blanks to complete the Next Permutation function.

DSA C#

public void NextPermutation(int[] nums) {
    int i = nums.Length - 2;
    while (i >= 0 && nums[i] [1] nums[i + 1]) {
        i--;
    }
    if (i >= 0) {
        int j = nums.Length - 1;
        while (j >= 0 && nums[j] [2] nums[i]) {
            j--;
        }
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }
    int start = i + 1;
    int end = nums.Length - 1;
    while (start [3] end) {
        int temp = nums[start];
        nums[start] = nums[end];
        nums[end] = temp;
        start++;
        end--;
    }
}

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

A>=

B<=

C<

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up comparison operators causes wrong permutation.

Not reversing the suffix correctly leads to incorrect output.

The first while uses '>=' to find the pivot, the second uses '<=' to find the swap candidate, and the last uses '<' to reverse the suffix.

Practice

1. What does the Next Permutation algorithm do to an array of numbers?

easy

A. It finds the maximum element in the array.

B. It sorts the array in ascending order.

C. It reverses the array.

D. It rearranges the array to the next lexicographically greater permutation.

Solution

1. Step 1: Understand the purpose of Next Permutation

   The algorithm finds the next arrangement of numbers that is just larger than the current one in dictionary order.

2. Step 2: Compare with other options

   Sorting or reversing changes the order but does not guarantee the next immediate permutation. Finding max element is unrelated.

3. Final Answer:

   It rearranges the array to the next lexicographically greater permutation. -> Option D

4. Quick Check:

   Next permutation = next lexicographically greater order [OK]

Hint: Next permutation means next bigger arrangement in order [OK]

Common Mistakes:

• Confusing next permutation with sorting
• Thinking it just reverses the array
• Assuming it finds max element

2. Which of the following C# code snippets correctly swaps two elements nums[i] and nums[j] in an array?

easy

A. int temp = nums[i]; nums[i] = nums[j]; nums[j] = temp;

B. nums[i] = nums[j]; nums[j] = nums[i];

C. Swap(nums[i], nums[j]);

D. nums[i], nums[j] = nums[j], nums[i];

Solution

1. Step 1: Check correct swap syntax in C#

   Using a temporary variable to hold one value before swapping is the correct approach in C#.

2. Step 2: Analyze other options

   nums[i] = nums[j]; nums[j] = nums[i]; overwrites nums[i] before assigning to nums[j]. Swap(nums[i], nums[j]); is invalid without a defined method. nums[i], nums[j] = nums[j], nums[i]; is Python syntax, not C#.

3. Final Answer:

   int temp = nums[i]; nums[i] = nums[j]; nums[j] = temp; -> Option A

4. Quick Check:

   Use temp variable to swap in C# [OK]

Hint: Use temp variable to swap values in C# [OK]

Common Mistakes:

• Overwriting one element before saving it
• Using Python tuple swap syntax in C#
• Calling undefined Swap method

3. What is the output array after applying the Next Permutation algorithm once on int[] nums = {1, 3, 2}?

medium

A. [1, 2, 3]

B. [3, 1, 2]

C. [2, 1, 3]

D. [1, 3, 2]

Solution

1. Step 1: Find the first decreasing element from the right

   From right, 2 < 3 is false, but 3 > 1 is true, so pivot is at index 0 (value 1).

2. Step 2: Find the element just larger than pivot to the right and swap

   Element just larger than 1 on right is 2 at index 2. Swap 1 and 2: array becomes [2, 3, 1].

3. Step 3: Reverse the suffix after pivot index

   Reverse elements after index 0: [3, 1] reversed to [1, 3]. Final array: [2, 1, 3].

4. Final Answer:

   [2, 1, 3] -> Option C

5. Quick Check:

   Next permutation of [1,3,2] = [2,1,3] [OK]

Hint: Swap pivot with next bigger, then reverse suffix [OK]

Common Mistakes:

• Not reversing suffix after swap
• Swapping with wrong element
• Confusing pivot position

4. Given the following code snippet for Next Permutation, what is the bug?

void NextPermutation(int[] nums) {
  int i = nums.Length - 2;
  while (i >= 0 && nums[i] >= nums[i + 1]) i--;
  if (i >= 0) {
    int j = nums.Length - 1;
    while (nums[j] <= nums[i]) j--;
    int temp = nums[i]; nums[i] = nums[j]; nums[j] = temp;
  }
  Array.Reverse(nums, i, nums.Length - i);
}

medium

A. The Array.Reverse call has wrong start index; should be i + 1.

B. The first while loop condition is incorrect.

C. Swapping elements uses wrong indices.

D. The method does not handle empty arrays.

Solution

1. Step 1: Analyze Array.Reverse parameters

   Array.Reverse(nums, i, nums.Length - i) reverses from index i, but suffix to reverse starts at i+1.

2. Step 2: Correct the start index for reversal

   It should be Array.Reverse(nums, i + 1, nums.Length - (i + 1)) to reverse the suffix properly.

3. Final Answer:

   The Array.Reverse call has wrong start index; should be i + 1. -> Option A

4. Quick Check:

   Reverse suffix starting at i+1, not i [OK]

Hint: Reverse suffix from i+1, not i [OK]

Common Mistakes:

• Reversing from wrong index
• Ignoring empty array edge case
• Swapping wrong elements

5. You have an array nums = {5, 4, 3, 2, 1}. What will be the result after applying the Next Permutation algorithm once, and why?

hard

A. [5, 4, 3, 2, 1] because it is already the highest permutation.

B. [1, 2, 3, 4, 5] because the array is in descending order, so next permutation is the lowest ascending order.

C. [5, 4, 3, 2, 2] because the last element is decreased by one.

D. [1, 5, 4, 3, 2] because the first element is swapped with the smallest.

Solution

1. Step 1: Identify if array is highest permutation

   The array is strictly descending, so it is the highest lexicographical permutation.

2. Step 2: Apply rule for highest permutation

   Next permutation resets to the lowest order by sorting ascending, resulting in [1, 2, 3, 4, 5].

3. Final Answer:

   [1, 2, 3, 4, 5] because the array is in descending order, so next permutation is the lowest ascending order. -> Option B

4. Quick Check:

   Descending array next permutation = ascending order [OK]

Hint: Descending array resets to ascending order as next permutation [OK]

Common Mistakes:

• Thinking highest permutation stays same
• Decreasing last element incorrectly
• Swapping wrong elements