Two pointers
Python Two Pointer Interview Questions
Two pointers means moving two positions through a sequence at once — from both ends, or at different speeds — instead of checking every possible pair. It is how a slow solution becomes a fast one when the input is sorted.
What two pointer questions ask you to do
Across the questions here, these are the shapes that come up:
- Find a pair of values that adds up to a target
- Remove duplicates from a sorted sequence in place
- Merge two sorted sequences
- Find a midpoint, or detect a loop
- Split a sequence around a value
Common mistakes with two pointers
It only works on a sorted sequence. Given unsorted input it returns a wrong answer rather than an error, and sorting first changes the cost of the whole solution, so that has to be part of the answer rather than a detail left out of it.
