334. Increasing Triplet Subsequence，tripletsubsequence

334. Increasing Triplet Subsequence，tripletsubsequence

Given an unsorted array return whether an increasing subsequence of length 3 exists or not in the array.

Formally the function should:

Return true if there exists i, j, k 
such that arr[i] < arr[j] < arr[k] given 0 ≤ i < j < k ≤ n-1 else return false.

Your algorithm should run in O(n) time complexity and O(1) space complexity.

Examples:
Given [1, 2, 3, 4, 5],
return true.

Given [5, 4, 3, 2, 1],
return false.

代碼如下：

 1 public class Solution {
 2     public boolean increasingTriplet(int[] nums) {
 3         
 4         if(nums.length<3)
 5         return false;
 6         
 7         for(int i=nums.length-1;i>=2;i--)
 8         {
 9             int j=i-1;
10             for( ;j>=1;j--)
11             {
12                 if(nums[i]>nums[j])
13                 {
14                        int k=j-1;
15                        for(;k>=0;k--)
16                        {
17                            if(nums[j]>nums[k])
18                            return true;
19                        }
20                 }
21                
22             }
23 
24         }
25         return false;
26     } 
27 }