Two Pointers

10 cards, about a minute. Read them the night before an interview, or right after the chapter to lock it in.

1. ◆what it is

   Two indices walk the data so every step throws away candidates for good: O(n) instead of the O(n²) all-pairs loop.

   #1
2. ↯use it when

   Sorted array + pair or triplet sum, palindrome check, reverse in place, or keep/remove in place: think two pointers.

   #2
3. { }template

   Opposite ends: l = 0, r = n - 1; while (l < r) — too small → l++, too big → r--.

   #3
4. { }template

   Same direction: read scans everything, write marks the next free slot and moves only for keepers.

   #4
5. { }template

   Two sorted inputs: one pointer each, take the smaller. Merging in place? Fill from the back so nothing is overwritten.

   #5
6. O( )cost

   O(n) time, O(1) extra space. Sorting first adds O(n log n), still far better than O(n²).

   #6
7. !trap

   l <= r pairs an element with itself. Pair problems use l < r.

   #7
8. !trap

   Sorting destroys original indices. If the answer needs them (LC 1 Two Sum), use a HashMap instead.

   #8
9. !trap

   3Sum: skip a duplicate i by comparing with the previous value, never the next one.

   #9
10. ✦pro tip

    Prove one move out loud: "even with the biggest value left, this one is too small, so it can never be in the answer."

    #10

Shaky on a card? Open the chapter or flip its recall cards.