Next Greater Element Using Stack in DSA Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the Next Greater Element (NGE) for an element in an array?

The Next Greater Element for an element is the first element to its right in the array that is greater than it. If no such element exists, the NGE is -1.

intermediate

Why do we use a stack to solve the Next Greater Element problem efficiently?

A stack helps keep track of elements for which we haven't found the next greater element yet. It allows us to process elements in a way that avoids repeated scanning, making the solution efficient (O(n) time).

intermediate

In the Next Greater Element algorithm using a stack, what do we do when the current element is greater than the element on the top of the stack?

We pop elements from the stack while the current element is greater than the top of the stack. For each popped element, the current element is its Next Greater Element.

beginner

What is the time complexity of the Next Greater Element algorithm using a stack?

The time complexity is O(n) because each element is pushed and popped at most once from the stack.

beginner

What will be the output Next Greater Elements for the array [4, 5, 2, 25]?

The Next Greater Elements are [5, 25, 25, -1]. Explanation: 4's NGE is 5, 5's NGE is 25, 2's NGE is 25, and 25 has no greater element to the right.

What does the stack store in the Next Greater Element algorithm?

A All elements in sorted order

B Indices or values of elements waiting for their next greater element

C Only the largest element in the array

D Elements that have found their next greater element

The stack stores elements (or their indices) for which we haven't found the next greater element yet.

If the current element is smaller than the top of the stack, what should we do?

A Push the current element onto the stack

B Pop the top element from the stack

C Ignore the current element

D Replace the top element with the current element

If the current element is smaller, we push it onto the stack to find its next greater element later.

What is the Next Greater Element for the last element in the array?

A The first element in the array

B The element itself

C The largest element in the array

D -1, since no elements are to the right

The last element has no elements to its right, so its Next Greater Element is -1.

What is the main advantage of using a stack for the Next Greater Element problem?

A It reduces time complexity to O(n)

B It sorts the array automatically

C It uses less memory than arrays

D It finds the smallest element quickly

Using a stack allows us to find Next Greater Elements in linear time, O(n), by avoiding repeated scanning.

What happens to elements left in the stack after processing the entire array?

A They are removed from the array

B They are the largest elements in the array

C They have no Next Greater Element, so their NGE is -1

D They become the Next Greater Elements for previous elements

Elements left in the stack have no greater element to their right, so their Next Greater Element is -1.

Explain how the stack is used step-by-step to find the Next Greater Element for each element in an array.

Think about how the stack helps track elements waiting for their next greater element.

Describe the time complexity of the Next Greater Element algorithm using a stack and why it is efficient.

Consider how many times each element interacts with the stack.

Practice

1. What does the Next Greater Element (NGE) problem using a stack find for each element in an array?

easy

A. The first greater element to the right of each element

B. The first smaller element to the left of each element

C. The sum of all elements greater than the current element

D. The last element in the array

Solution

1. Step 1: Understand the problem goal

   The Next Greater Element problem asks to find for each element the first element to its right that is greater than it.

2. Step 2: Recognize the stack usage

   A stack helps track elements for which we haven't found a greater element yet, efficiently finding the next greater element to the right.

3. Final Answer:

   The first greater element to the right of each element -> Option A

4. Quick Check:

   NGE = first greater element on right [OK]

Hint: Look right for first bigger number using stack [OK]

Common Mistakes:

• Confusing next greater with next smaller element
• Looking to the left instead of right
• Thinking it sums elements instead of finding one

2. Which Go code snippet correctly initializes an empty stack to use for Next Greater Element logic?

easy

A. stack := 0

B. stack := make(map[int]int)

C. stack := new(int)

D. stack := []int{}

Solution

1. Step 1: Identify stack data structure in Go

   Stacks can be implemented using slices in Go. An empty stack is an empty slice of ints: []int{}.

2. Step 2: Check other options

   stack := make(map[int]int) is a map, not a stack. stack := new(int) is a pointer to int, not a stack. stack := 0 is an int variable, not a stack.

3. Final Answer:

   stack := []int{} -> Option D

4. Quick Check:

   Stack as slice = []int{} [OK]

Hint: Use empty int slice for stack in Go [OK]

Common Mistakes:

• Using map instead of slice for stack
• Using pointer instead of slice
• Initializing stack as int variable

3. What is the output of the Next Greater Element function for the input array [4, 5, 2, 25]?

medium

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

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

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

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

Solution

1. Step 1: Trace the array elements with stack

   For 4, next greater is 5; for 5, next greater is 25; for 2, next greater is 25; for 25, no greater element, so -1.

2. Step 2: Form the output array

   Output is [5, 25, 25, -1].

3. Final Answer:

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

4. Quick Check:

   Next greater elements found correctly [OK]

Hint: Check each element's right side for bigger number [OK]

Common Mistakes:

• Assigning -1 too early
• Mixing up indices in output
• Ignoring last element's -1

4. Given the following Go code snippet for Next Greater Element, what is the bug?

func nextGreater(arr []int) []int {
  stack := []int{}
  result := make([]int, len(arr))
  for i := len(arr) - 1; i >= 0; i-- {
    for len(stack) > 0 && stack[len(stack)-1] <= arr[i] {
      stack = stack[:len(stack)-1]
    }
    if len(stack) == 0 {
      result[i] = 0
    } else {
      result[i] = stack[len(stack)-1]
    }
    stack = append(stack, arr[i])
  }
  return result
}

medium

A. Stack is not initialized properly

B. Loop iterates in wrong direction

C. Result uses 0 instead of -1 for no greater element

D. Comparison operator should be >= instead of <=

Solution

1. Step 1: Identify the no greater element marker

   The code assigns 0 when no greater element is found, but the standard is to use -1.

2. Step 2: Check other parts

   Stack initialization and loop direction are correct. Comparison operator <= is correct for popping smaller or equal elements.

3. Final Answer:

   Result uses 0 instead of -1 for no greater element -> Option C

4. Quick Check:

   No greater element should be -1 [OK]

Hint: Use -1 for no next greater element, not 0 [OK]

Common Mistakes:

• Returning 0 instead of -1 for no greater element
• Changing loop direction incorrectly
• Wrong comparison operator causing wrong pops

5. You have an array [13, 7, 6, 12]. Using the Next Greater Element stack approach, what is the output array?

hard

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

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

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

D. [ 7, 6, 12, -1 ]

Solution

1. Step 1: Find next greater for each element

   For 13, no greater element to right, so -1. For 7, next greater is 12. For 6, next greater is 12. For 12, no greater element, so -1.

2. Step 2: Write output array

   Output is [-1, 12, 12, -1].

3. Final Answer:

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

4. Quick Check:

   Check right side for bigger numbers [OK]

Hint: Check each element's right side for bigger number [OK]

Common Mistakes:

• Assigning wrong next greater for 7 and 6
• Confusing output with input order
• Using 0 instead of -1 for no greater element