Data Structures & Algorithms·Reason about cost and structure

Arrays, strings and two pointers

Contiguous storage is fast; index discipline is everything.

Intermediate 14 min read Lesson 7 of 300 Challenge included

What you will be able to do

Prefer linear scans when the data is already ordered.
Use two pointers for sorted pair and window problems.
Be explicit about whether indexes are inclusive or exclusive.

In plain English

Arrays give O(1) indexed access and O(n) insertion in the middle. Two-pointer techniques turn some nested scans into a single pass. Strings are sequences with their own encoding and immutability rules.

two-pointers.jsJavaScript

function hasPairSum(sorted, target) {
  let left = 0
  let right = sorted.length - 1

  while (left < right) {
    const sum = sorted[left] + sorted[right]
    if (sum === target) return true
    if (sum < target) left += 1
    else right -= 1
  }
  return false
}

Your turn

Reverse in place

Medium

Reverse an array of characters in place using two pointers.

JS · starter

function reverse(chars) {}

Check your understanding before moving on.

Knowledge checkQuestion 1

Why does the two-pointer pair-sum algorithm rely on sorted input?

Select an answer to continue

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Lesson 7 of 300 · 0% of course