First and Last Occurrence of Element in DSA Typescript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Find first and last occurrence of 3 in a sorted array

What is the output of the following TypeScript code that finds the first and last occurrence of the number 3 in the array?

DSA Typescript

function firstAndLastOccurrence(arr: number[], target: number): [number, number] {
  let first = -1;
  let last = -1;
  for (let i = 0; i < arr.length; i++) {
    if (arr[i] === target) {
      if (first === -1) first = i;
      last = i;
    }
  }
  return [first, last];
}

const arr = [1, 2, 3, 3, 3, 4, 5];
console.log(firstAndLastOccurrence(arr, 3));

A [2, 4]

B [1, 3]

C [3, 5]

D [-1, -1]

💡 Hint

Look for the first and last index where the target appears in the array.

The function scans the array once. It sets 'first' when it finds the target the first time and updates 'last' every time it finds the target. For target 3, first occurrence is at index 2 and last at index 4.

❓ Predict Output

intermediate

Output when target is not in the array

What will the output be when searching for 6 in the array using the same function?

DSA Typescript

function firstAndLastOccurrence(arr: number[], target: number): [number, number] {
  let first = -1;
  let last = -1;
  for (let i = 0; i < arr.length; i++) {
    if (arr[i] === target) {
      if (first === -1) first = i;
      last = i;
    }
  }
  return [first, last];
}

const arr = [1, 2, 3, 3, 3, 4, 5];
console.log(firstAndLastOccurrence(arr, 6));

A [0, 0]

B [-1, -1]

C [6, 6]

D [null, null]

💡 Hint

If the target is not found, the function returns [-1, -1].

Since 6 is not in the array, 'first' and 'last' remain -1, so the function returns [-1, -1].

🔧 Debug

advanced

Identify the error in binary search for first occurrence

What error does the following TypeScript code produce when trying to find the first occurrence of a target using binary search?

DSA Typescript

function firstOccurrence(arr: number[], target: number): number {
  let low = 0;
  let high = arr.length - 1;
  let result = -1;
  while (low <= high) {
    let mid = Math.floor((low + high) / 2);
    if (arr[mid] === target) {
      result = mid;
      high = mid - 1; // Move left to find first occurrence
    } else if (arr[mid] < target) {
      low = mid + 1;
    } else {
      high = mid - 1;
    }
  }
  return result;
}

console.log(firstOccurrence([1,2,3,3,3,4,5], 3));

A Infinite loop

B Returns -1 always

C SyntaxError due to missing semicolon

D Returns 4 (last occurrence) instead of first

💡 Hint

Check how 'high' is updated when target is found.

The original code incorrectly moves 'high' to mid + 1, which searches right side, so it finds last occurrence instead of first.

❓ Predict Output

advanced

Output of last occurrence binary search

What is the output of this code that finds the last occurrence of 3 in the array using binary search?

DSA Typescript

function lastOccurrence(arr: number[], target: number): number {
  let low = 0;
  let high = arr.length - 1;
  let result = -1;
  while (low <= high) {
    let mid = Math.floor((low + high) / 2);
    if (arr[mid] === target) {
      result = mid;
      low = mid + 1; // Move right to find last occurrence
    } else if (arr[mid] < target) {
      low = mid + 1;
    } else {
      high = mid - 1;
    }
  }
  return result;
}

console.log(lastOccurrence([1,2,3,3,3,4,5], 3));

A 4

B 2

C -1

D 3

💡 Hint

The function moves 'low' to mid + 1 when target is found to find last occurrence.

The last occurrence of 3 is at index 4, so the function returns 4.

🧠 Conceptual

expert

Time complexity of finding first and last occurrence using binary search

What is the time complexity of finding both the first and last occurrence of an element in a sorted array using binary search?

A O(n log n)

B O(n)

C O(log n)

D O(1)

💡 Hint

Each binary search takes logarithmic time, and you do two searches.

Finding first and last occurrence requires two binary searches, each O(log n), so total is O(log n).

Practice

1. What does the "First and Last Occurrence of Element" problem ask you to find in an array?

easy

A. The maximum element in the array

B. The sum of all elements in the array

C. The first and last positions where a given element appears

D. The number of unique elements in the array

Solution

1. Step 1: Understand the problem statement

   The problem asks to find the first and last index positions of a specific element in an array.

2. Step 2: Identify what is required

   We need to locate where the element appears first and last, not sums or counts.

3. Final Answer:

   The first and last positions where a given element appears -> Option C

4. Quick Check:

   First and last occurrence = positions of element [OK]

Hint: Look for positions of element, not sums or counts [OK]

Common Mistakes:

• Confusing occurrence with sum or max
• Thinking about unique elements instead
• Ignoring the order of elements

2. Which of the following TypeScript function signatures correctly represents a function to find first and last occurrence of a number in an array?

easy

A. function findOccurrences(arr: number[], target: number): number

B. function findOccurrences(arr: string[], target: string): number

C. function findOccurrences(arr: number, target: number): number[]

D. function findOccurrences(arr: number[], target: number): [number, number]

Solution

1. Step 1: Analyze the function parameters

   The function should take an array of numbers and a target number to find.

2. Step 2: Analyze the return type

   We want to return two numbers (first and last index), so a tuple [number, number] is appropriate.

3. Final Answer:

   function findOccurrences(arr: number[], target: number): [number, number] -> Option D

4. Quick Check:

   Input array and target, output tuple of two indexes [OK]

Hint: Return a tuple of two numbers for first and last indexes [OK]

Common Mistakes:

• Using wrong input types like single number instead of array
• Returning only one number instead of two
• Returning array of wrong type

3. What will be the output of the following TypeScript code?

function findOccurrences(arr: number[], target: number): [number, number] {
  let first = -1;
  let last = -1;
  for (let i = 0; i < arr.length; i++) {
    if (arr[i] === target) {
      if (first === -1) first = i;
      last = i;
    }
  }
  return [first, last];
}

console.log(findOccurrences([5, 3, 7, 3, 9], 3));

medium

A. [1, 3]

B. [3, 3]

C. [0, 4]

D. [-1, -1]

Solution

1. Step 1: Trace the loop to find first occurrence

   At index 1, element is 3 which matches target, so first = 1.

2. Step 2: Update last occurrence while looping

   At index 3, element is 3 again, so last = 3.

3. Final Answer:

   [1, 3] -> Option A

4. Quick Check:

   First = 1, Last = 3 [OK]

Hint: Track first when found, update last every time [OK]

Common Mistakes:

• Returning first occurrence twice
• Returning indexes of wrong elements
• Not updating last occurrence properly

4. Identify the bug in this TypeScript function to find first and last occurrence:

function findOccurrences(arr: number[], target: number): [number, number] {
  let first = 0;
  let last = 0;
  for (let i = 0; i < arr.length; i++) {
    if (arr[i] === target) {
      if (first === 0) first = i;
      last = i;
    }
  }
  return [first, last];
}

medium

A. last should be initialized to -1 instead of 0

B. first initialized to 0 causes wrong first occurrence when target is at index 0

C. Loop should start from 1 instead of 0

D. Missing return type annotation

Solution

1. Step 1: Check initialization of first

   first is set to 0, so if target is at index 0, condition first === 0 is true initially, causing incorrect logic.

2. Step 2: Understand impact on first occurrence

   If target is at index 0, first will never update correctly because first === 0 is true initially, so first stays 0 even if target not found.

3. Final Answer:

   first initialized to 0 causes wrong first occurrence when target is at index 0 -> Option B

4. Quick Check:

   Initialize first to -1 to fix [OK]

Hint: Initialize first to -1 to detect if target found [OK]

Common Mistakes:

• Initializing first to 0 instead of -1
• Not handling target at index 0 correctly
• Assuming first occurrence is always > 0

5. Given a sorted array with possible duplicates, which approach is the most efficient to find the first and last occurrence of a target element?

hard

A. Use two binary searches: one for first occurrence, one for last occurrence

B. Traverse the array from start to end to find first and last occurrence

C. Sort the array again and then find the element

D. Use a hash map to store all occurrences and then find first and last

Solution

1. Step 1: Understand the array is sorted

   Since the array is sorted, binary search can be used to find elements efficiently.

2. Step 2: Use binary search twice

   One binary search finds the first occurrence, another finds the last occurrence, both in O(log n) time.

3. Step 3: Compare with other options

   Traversing is O(n), sorting again is unnecessary, hash map uses extra space and is less efficient.

4. Final Answer:

   Use two binary searches: one for first occurrence, one for last occurrence -> Option A

5. Quick Check:

   Binary search twice = efficient for sorted arrays [OK]

Hint: Use binary search twice for first and last positions [OK]

Common Mistakes:

• Using linear search on sorted array
• Sorting array again unnecessarily
• Using extra space with hash map