leetcode-456. 132 Pattern

一、问题描述

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: FalseExplanation: There is no 132 pattern in the sequence.

Example 2:

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

Example 3:

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

二、思路分析

用一个栈，遍历数组从尾到头，维持栈顶为当前最大值，并且用一个数s保持压入最大值之前栈顶的元素，如果当前值nums[i]小于s且栈不为空，则有132.

三、代码

class Solution {public:    bool find132pattern(vector<int>& nums) {        int s2 = -2147483648;        stack<int> temp;        for(int i = nums.size() - 1; i >= 0; i--) {            if (nums[i] < s2) return true;            while (!temp.empty() && nums[i] > temp.top()) {                s2 = temp.top();                temp.pop();            }            temp.push(nums[i]);        }        return false;    }};