132-pattern.py

# Time:  O(n)
# Space: O(n)

# Given a sequence of n integers a1, a2, ..., an,
# a 132 pattern is a subsequence ai, aj, ak such that i < j < k and
# ai < ak < aj. Design an algorithm that takes a list of n numbers as
# input and checks whether there is a 132 pattern in the list.
#
# Note: n will be less than 15,000.
#
# Example 1:
# Input: [1, 2, 3, 4]
#
# Output: False
#
# Explanation: There is no 132 pattern in the sequence.
# Example 2:
# Input: [3, 1, 4, 2]
#
# Output: True
#
# Explanation: There is a 132 pattern in the sequence: [1, 4, 2].
# Example 3:
# Input: [-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].

class Solution(object):
    def find132pattern(self, nums):
        """
        :type nums: List[int]
        :rtype: bool
        """
        ak = float("-inf")
        st = []
        for i in reversed(xrange(len(nums))):
            if nums[i] < ak:
                return True
            else:
                while st and nums[i] > st[-1]:
                    ak = st.pop()
            st.append(nums[i])
        return False