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456. 132 Pattern.go
456. 132 Pattern_test.go

# 456. 132 Pattern

## 题目

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].
``````

## 解题思路

• 这一题用暴力解法一定超时
• 这一题算是单调栈的经典解法，可以考虑从数组末尾开始往前扫，维护一个递减序列
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