Leetcode-456. 132 Pattern

前言：正好碰见Leetcode有一次在线笔试，测试一下，Rank：92/807。如有错误欢迎指正。博主首发CSDN，mcf171专栏。

博客链接：mcf171的博客
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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].

这个题目错了好几次，原因如下：

1、是严格的小于

2、循环判断条件写错了

这个题目解的也不是很好时间复杂度O(n^2)

public class Solution {
    public boolean find132pattern(int[] nums) {
        if(nums.length < 3) return false;
        boolean flag = false;
        for(int k = nums.length - 1; k >=2; k --){
            int i = 0, j = k -1;
            while(i < j){
                if(nums[i] < nums[k] && nums[k] < nums[j]){flag = true;break;}
                if(nums[i] >=nums[k]) i ++;
                if(nums[j] <= nums[k]) j--;
            }
            if(flag) break;
        }

        return flag;
    }
}