334. Increasing Triplet Subsequence

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.

代码如下：

 public class Solution {
     public boolean increasingTriplet(int[] nums) {

         if(nums.length<3)
         return false;

         for(int i=nums.length-1;i>=2;i--)
         {
             int j=i-1;
             for( ;j>=1;j--)
             {
                 if(nums[i]>nums[j])
                 {
                        int k=j-1;
                        for(;k>=0;k--)
                        {
                            if(nums[j]>nums[k])
                            return true;
                        }
                 }

             }

         }
         return false;
     }
 }