Identify

• when searching pairs in a sorted array or linked list
• to compare each element of an array to its other elements
• when dealing with sorted arrays (or Linked Lists) and need to find a set of elements that fulfill certain constraints
• when the set of elements in the array is a pair, a triplet, or even a subarray
• when there is a target value to match or duplicates to be removed
• sorted arrays

Motive

• two pointers iterate through the data structure in tandem until one or both of the pointers hit a certain condition
• two pointers are needed because with just a pointer, you would have to continually loop back through the array to find the answer and this back-and-forth with a single iterator is inefficient for time and space complexity
• while the brute force or naive solution with 1 pointer would work, it will produce something along the lines of O(n²), while two pointers can help you find a solution with better space O(1) or runtime complexity O(n)

Steps

• initiate two pointers: left, right = 0, len(nums) - 1
• iterate: while left < right:
• increase depending on the problem

Complexity

Time O(n)

Space O(1)

Common Problems