456. 132 Pattern

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].

思路：
132可以看作是以第三个数为基准，一个数大于他一个数小于他，而且对两个数的位置有要求，更小的在前。从数组末尾开始遍历。

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