456. 132 Pattern

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.

Follow up: The O(n^2) is trivial, could you come up with the O(n logn) or the O(n) solution?

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 <= 104
• -109 <= nums[i] <= 109

# @lc code=start
using LeetCode

function find132pattern(nums::Vector{Int})
    stk = Int[]
    second_great = typemin(Int)
    for num in Iterators.reverse(nums)
        second_great > num && return true
        while !isempty(stk) && stk[end] < num
            second_great = pop!(stk)
        end
        push!(stk, num)
    end
    return false
end
# @lc code=end

find132pattern (generic function with 1 method)

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