132 Pattern

MediumStackmonotonic stack

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Problem

Given an integer array nums, a **132 pattern** is a triple of indices i < j < k such that nums[i] < nums[k] < nums[j]: the first value is the smallest, the middle value is the largest, and the last value lies strictly between them. Return true if nums contains at least one 132 pattern, otherwise false. All comparisons are strict, so equal values never complete a pattern on their own. Aim for O(n) time.

Examples

Input: nums = [3,5,0,3,4]
Output: true
Indices 0, 1, 4 give 3 < 4 < 5, which is a 132 pattern.
Input: nums = [1,2,3,4]
Output: false
The array is strictly increasing, so no later value can fall between an earlier pair.
Input: nums = [-2,1,-1]
Output: true
Take i = 0, j = 1, k = 2: nums[i] = -2 < nums[k] = -1 < nums[j] = 1.

Constraints

  • 1 <= nums.length <= 2 * 10^5
  • -10^9 <= nums[i] <= 10^9

What the interviewer grades

A correct solution is one of five scores. The report also grades problem understanding (did you clarify inputs and edge cases before coding), problem solving, communication while you work, code quality, and testing. See a sample report, or read up on the pattern in the lessons.

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