456. 132 Pattern

Medium

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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: false
Explanation: There is no 132 pattern in the sequence.

Example 2:

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

Example 3:

Input: nums = [-1,3,2,0]
Output: true
Explanation: 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

class Solution:
    def find132pattern(self, nums: List[int]) -> bool:
        # Pattern s1 s2 s3
        s3 = -float("inf")
        # Stack contains all s2
        stack = []
        for index in range(len(nums)-1, -1, -1):
            # This Condition denotes that we have found all 3 elements
            if nums[index] < s3:
                return True
            while stack and stack[-1] < nums[index]:
                s3 = stack.pop()
            stack.append(nums[index])
        return False