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Two Pointers

Two indices moving under an invariant. Converts nested loops into one pass.

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What you actually need to hold in your head

  • Opposite ends for sorted pair problems
  • Same direction for partitioning
  • Fast and slow for cycles and midpoints
  • Each pointer only moves forward, so O(n)
Comfortable with these first
Arrays18%Sorting18%
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Watch it run

Step through the algorithm before you write it.

Two PointersE

On a sorted array, one pointer at each end. Move the pointer that can only push the sum in the direction you need.

O(n)

Patterns in this topic

The trigger phrases are what you are training yourself to notice.

Two Pointers (opposite ends)O(n) after sorting

Start one pointer at each end. Compare the current value against the target and move the pointer that can only improve the result.

sorted arraypair or triplet summing to target

Two Pointers problems

Ordered by sheet phase, so the early ones are the highest signal.

0 / 11 solved
1
3Sum
MTwo Pointers
2
Container With Most Water
MTwo Pointers
3
Trapping Rain Water
HTwo Pointers
4
Merge Sorted Array
ETwo Pointers
5
Move Zeroes
ETwo Pointers
6
Sort Colors
MTwo Pointers
7
Two Sum II - Input Array Is Sorted
MTwo Pointers
8
Remove Duplicates from Sorted Array
ETwo Pointers
9
3Sum Closest
MTwo Pointers
10
4Sum
MTwo Pointers
11
Squares of a Sorted Array
ETwo Pointers