First and Last Occurrence of Element in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to find the first occurrence index of a target element in an array.

DSA Typescript

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

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

Aarr

Bi

Ctarget

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'i' instead of 'target' in the comparison.

Comparing arr[i] with arr instead of target.

We compare each element arr[i] with the target to find the first occurrence index.

2fill in blank

medium

Complete the code to find the last occurrence index of a target element in an array.

DSA Typescript

function lastOccurrence(arr: number[], target: number): number {
  for (let i = arr.length - 1; i >= 0; i--) {
    if (arr[i] === [1]) {
      return i;
    }
  }
  return -1;
}

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

Atarget

Barr

Ci

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'arr' instead of 'target' in the comparison.

Looping forward instead of backward.

We compare each element arr[i] with the target starting from the end to find the last occurrence index.

3fill in blank

hard

Fix the error in the binary search code to find the first occurrence of target in a sorted array.

DSA Typescript

function firstOccurrenceBinarySearch(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 = [1];
    } else if (arr[mid] < target) {
      low = mid + 1;
    } else {
      high = mid - 1;
    }
  }
  return result;
}

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Amid + 1

Blow + 1

Chigh - 1

Dmid - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Setting high = mid + 1 which moves right instead of left.

Setting low incorrectly after finding target.

To find the first occurrence, after finding target at mid, we move high to mid - 1 to search left side.

4fill in blank

hard

Fill both blanks to complete the binary search code to find the last occurrence of target in a sorted array.

DSA Typescript

function lastOccurrenceBinarySearch(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 = [1];
    } else if (arr[mid] < target) {
      low = mid + 1;
    } else {
      high = [2];
    }
  }
  return result;
}

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Amid + 1

Bmid - 1

Chigh - 1

Dlow + 1

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping low and high updates.

Not updating low or high correctly after finding target.

To find last occurrence, after finding target at mid, move low to mid + 1 to search right side; else adjust high to mid - 1.

5fill in blank

hard

Fill all three blanks to create a function that returns both first and last occurrence indices of target in a sorted array using binary search.

DSA Typescript

function firstAndLastOccurrence(arr: number[], target: number): {first: number, last: number} {
  let low = 0, high = arr.length - 1;
  let first = -1, last = -1;
  while (low <= high) {
    let mid = Math.floor((low + high) / 2);
    if (arr[mid] === target) {
      first = mid;
      high = [1];
    } else if (arr[mid] < target) {
      low = mid + 1;
    } else {
      high = mid - 1;
    }
  }
  low = 0;
  high = arr.length - 1;
  while (low <= high) {
    let mid = Math.floor((low + high) / 2);
    if (arr[mid] === target) {
      last = mid;
      low = [2];
    } else if (arr[mid] < target) {
      low = mid + 1;
    } else {
      high = [3];
    }
  }
  return {first, last};
}

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

Amid - 1

Bmid + 1

Chigh - 1

Dlow + 1

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up low and high updates in the two searches.

Not resetting low and high before second search.

For first occurrence, move high to mid - 1; for last occurrence, move low to mid + 1; else adjust high to mid - 1.