動機

久違的leetcode

Problem

Given an array of n integers nums, a 132 pattern is a subsequence of three integers nums[i], nums[j] and nums[k] such that i < j < k and nums[i] < nums[k] < nums[j].

Return true if there is a 132 pattern in nums, otherwise, return false.

 

Example 1:

Input: nums = [1,2,3,4]Output: falseExplanation: There is no 132 pattern in the sequence.

Example 2:

Input: nums = [3,1,4,2]Output: trueExplanation: There is a 132 pattern in the sequence: [1, 4, 2].

Example 3:

Input: nums = [-1,3,2,0]Output: trueExplanation: There are three 132 patterns in the sequence: [-1, 3, 2], [-1, 3, 0] and [-1, 2, 0].

 

Constraints:

  • n == nums.length
  • 1 <= n <= 2 * 105
  • -109 <= nums[i] <= 109

Sol

class Solution:
    def find132pattern(self, nums: List[int]) -> bool:
        stk = []
        mid = float('-inf')
        for n in reversed(nums):
            if n < mid and stk:
                return True
            
            while stk and stk[-1] < n:
                mid = max(mid,stk.pop())
            stk += n,
        return False