Next Greater Element Using Stack in DSA Rust - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize an empty stack for integers.

DSA Rust

let mut stack: Vec<i32> = Vec::[1]();

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Awith_capacity

Bnew

Cdefault

Dfrom

Attempts:

3 left

💡 Hint

Common Mistakes

Using with_capacity without specifying capacity.

Using from which requires an initial collection.

We use Vec::new() to create an empty vector which acts as a stack.

2fill in blank

medium

Complete the code to push an element onto the stack.

DSA Rust

stack.[1](num);

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Aappend

Binsert

Cpop

Dpush

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop which removes elements.

Using append which adds multiple elements.

Use push to add an element to the top of the stack.

3fill in blank

hard

Fix the error in the condition to check if the stack is not empty.

DSA Rust

while !stack.[1]().is_empty() && stack.last().unwrap() < num {

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Alen

Blast

Cis_empty

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop() which removes elements.

Using last() which returns an element, not a boolean.

Use stack.len() to check if the stack is not empty.

4fill in blank

hard

Fill both blanks to assign the next greater element and push the current index.

DSA Rust

result[i] = if stack.is_empty() [1] else [2];
stack.push(i);

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A-1

Bstack.last().unwrap().clone()

Cstack.pop().unwrap()

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using stack.pop() which removes the element.

Assigning 0 instead of -1 when stack is empty.

If the stack is empty, no greater element exists, so assign -1. Otherwise, assign the index at the top of the stack.

5fill in blank

hard

Fill all three blanks to complete the loop that finds the next greater element for each number.

DSA Rust

for [1] in (0..nums.len()).rev() {
    let num = nums[[2]];
    while !stack.is_empty() && nums[*stack.last().unwrap()] <= num {
        stack.pop();
    }
    result[[3]] = if stack.is_empty() { -1 } else { nums[*stack.last().unwrap()] };
    stack.push([1]);
}

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Ai

Bnum

Dj

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variables for loop and indexing causing errors.

Using num as an index which is incorrect.

The loop variable i is used to index the array and result vector consistently.

Practice

1. What is the main purpose of using a stack in the Next Greater Element problem?

easy

A. To keep track of elements for which we haven't found the next greater element yet

B. To sort the array elements in ascending order

C. To store the final output of next greater elements

D. To reverse the array elements before processing

Solution

1. Step 1: Understand the role of stack in the problem

   The stack stores elements that are waiting to find their next greater element as we scan the array.

2. Step 2: Clarify what the stack does not do

   The stack does not sort or store final output; it only helps track elements pending next greater element discovery.

3. Final Answer:

   To keep track of elements for which we haven't found the next greater element yet -> Option A

4. Quick Check:

   Stack tracks pending elements = A [OK]

Hint: Stack holds candidates waiting for next greater element [OK]

Common Mistakes:

• Thinking stack sorts the array
• Confusing stack with output storage
• Assuming stack reverses the array

2. Which of the following Rust code snippets correctly initializes an empty stack to use for Next Greater Element?

easy

A. let mut stack = Vec::empty();

B. let mut stack: Vec<i32> = Vec::new();

C. let stack = new Vec<i32>();

D. let stack = Vec<i32>;

Solution

1. Step 1: Identify correct Rust syntax for empty vector

   In Rust, an empty vector is created with Vec::new() and mutable if we want to push elements.

2. Step 2: Check each option

   let mut stack: Vec<i32> = Vec::new(); uses correct syntax with mut and Vec::new(). Options A, B, and C are invalid Rust syntax.

3. Final Answer:

   let mut stack: Vec<i32> = Vec::new(); -> Option B

4. Quick Check:

   Vec::new() with mut = D [OK]

Hint: Use let mut stack: Vec<i32> = Vec::new(); to create empty stack [OK]

Common Mistakes:

• Omitting mut when pushing elements
• Using incorrect Vec initialization syntax
• Confusing Vec::new() with other methods

3. What is the output of the following Rust code for Next Greater Element?

let nums = vec![4, 5, 2, 25];
let mut stack: Vec<usize> = Vec::new();
let mut result = vec![-1; nums.len()];

for i in 0..nums.len() {
    while !stack.is_empty() && nums[*stack.last().unwrap()] < nums[i] {
        let idx = stack.pop().unwrap();
        result[idx] = nums[i];
    }
    stack.push(i);
}

println!("{:?}", result);

medium

A. [-1, 25, 25, -1]

B. [25, -1, 5, -1]

C. [5, 25, 25, -1]

D. [5, 2, 25, -1]

Solution

1. Step 1: Trace the loop and stack operations

   Start with empty stack and result [-1, -1, -1, -1]. - i=0: stack empty, push 0 (stack=[0]) - i=1: nums[0]=4 < nums[1]=5, pop 0, result[0]=5, push 1 (stack=[1]) - i=2: nums[1]=5 > nums[2]=2, push 2 (stack=[1,2]) - i=3: nums[2]=2 < nums[3]=25, pop 2, result[2]=25 nums[1]=5 < 25, pop 1, result[1]=25, push 3 (stack=[3])

2. Step 2: Finalize result array

   Stack has index 3 left, result[3] remains -1. Final result is [5, 25, 25, -1].

3. Final Answer:

   [5, 25, 25, -1] -> Option C

4. Quick Check:

   Next greater elements = [5, 25, 25, -1] [OK]

Hint: Pop smaller indices until current is smaller, assign next greater [OK]

Common Mistakes:

• Not updating result when popping from stack
• Pushing indices instead of values
• Confusing indices and values in comparisons

4. Identify the bug in this Rust code snippet for Next Greater Element and select the fix:

let nums = vec![2, 7, 3, 5, 4, 6, 8];
let mut stack: Vec<i32> = Vec::new();
let mut result = vec![-1; nums.len()];

for i in 0..nums.len() {
    while !stack.is_empty() && nums[stack.last().unwrap()] < nums[i] {
        let idx = stack.pop().unwrap();
        result[idx] = nums[i];
    }
    stack.push(i as i32);
}

println!("{:?}", result);

medium

A. Change stack type to Vec<usize> and remove i32 casts

B. Change result vector to Vec<i32> with zeros instead of -1

C. Replace while loop with if statement

D. Push nums[i] instead of index i into stack

Solution

1. Step 1: Identify type mismatch in stack usage

   The stack stores indices to access nums. Indices are usize, but stack is Vec<i32> and i is cast to i32. This causes type mismatch and indexing errors.

2. Step 2: Fix by using correct type for stack

   Change stack to Vec<usize> and push i directly without casting. This matches indexing type and avoids errors.

3. Final Answer:

   Change stack type to Vec<usize> and remove i32 casts -> Option A

4. Quick Check:

   Stack indices must be usize = A [OK]

Hint: Use usize for indices in stack to match array indexing [OK]

Common Mistakes:

• Using i32 for indices causing type errors
• Pushing values instead of indices
• Replacing while with if losing multiple pops

5. Given the array [13, 7, 6, 12], apply the Next Greater Element using stack approach and provide the final output array.

hard

A. [ -1, 13, 12, -1 ]

B. [ 12, -1, -1, -1 ]

C. [ 7, 12, 13, -1 ]

D. [ -1, 12, 12, -1 ]

Solution

1. Step 1: Initialize stack and result

   Start with empty stack and result = [-1, -1, -1, -1]. Process elements left to right.

2. Step 2: Process each element and update result

   - i=0 (13): stack empty, push 0 (stack=[0]) - i=1 (7): 13 > 7, push 1 (stack=[0,1]) - i=2 (6): 7 > 6, push 2 (stack=[0,1,2]) - i=3 (12): 6 < 12, pop 2, result[2]=12 7 < 12, pop 1, result[1]=12 13 > 12, push 3 (stack=[0,3])

3. Step 3: Finalize result

   Stack has indices 0 and 3 left, their result remains -1. Final result: [-1, 12, 12, -1]

4. Final Answer:

   [-1, 12, 12, -1] -> Option D

5. Quick Check:

   Next greater elements = [-1, 12, 12, -1] [OK]

Hint: Pop smaller elements until current is smaller, assign next greater [OK]

Common Mistakes:

• Assigning wrong next greater for first element
• Not popping all smaller elements before pushing
• Confusing values and indices in stack