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LEETFLIX

Pancake Sorting

MediumArrayTwo pointersGreedySorting
Solve on LeetCode

Medium · two pointers solution for Pancake Sorting. Python, Go, and a step-by-step animation.

01

Problem

Read the prompt, flip through the examples, then steal the idea.

LeetCode 969. Pancake Sorting. The copyrighted prompt is on LeetCode. This page is a working two pointers solution for this problem: editable input, Python, Go, and a step-by-step animation.

Constraints

  • —Player inputs are capped so the animation stays readable.
  • —Match LeetCode's own constraints when you submit there.

Examples

Input

nums =
[2, 7, 11, 15]
target =
9

Output

[0, 1]

2 + 7 = 9. The inward walk finds the pair in one comparison.

02

Solution

Python and Go. The walkthrough below runs this same code.

Approach 1Optimal▶ Animated

Pancake Sorting — two pointers

On a sorted array the pair that sums to target is found by walking inward. Built for LeetCode 969. Pancake Sorting.

If the current sum is too small, the only way to grow it is to advance lo. If it is too big, retreat hi. Each step discards an index forever, so the walk is linear.

Algorithm

  1. Set lo = 0, hi = n−1.
  2. While lo < hi, compare nums[lo] + nums[hi] to target.
  3. Equal → return the indices. Too small → lo++. Too big → hi--.
Time
O(n)
Space
O(1)

Assumes the array is sorted.

Solution.py
class Solution:    def pancakeSorting(self, nums: list[int], target: int) -> list[int]:        lo, hi = 0, len(nums) - 1        while lo < hi:            s = nums[lo] + nums[hi]            if s == target:                return [lo, hi]            if s < target:                lo += 1            else:                hi -= 1        return []
Watch this run

03

Interactive walkthrough

Playing “Pancake Sorting — two pointers”. Edit the input, hit Run, then scrub.

Approach: Pancake Sorting — two pointers

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