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# 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.
```

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

## Example

**Example 1:**

```
Input: [1,2,3,4,5]
Output: true
```

**Example 2:**

```
Input: [5,4,3,2,1]
Output: false
```

## Note

常数个数版本单调栈，主要体会一下思路

通过观察这题的具体性质，我们发现在这里我们所谓的 "单调栈" 长度其实是固定的，就是3，等于3了直接返回就行。

因为 3 非常小，我们只需要存两个值，分别代表着单调栈的第一个和第二个位置；当我们碰到第三个位置的情况，就可以返回 true 了。

## Code

```java
class Solution {
    public boolean increasingTriplet(int[] nums) {
        int small = Integer.MAX_VALUE;
        int big = small;
        for (int num : nums) {
            if (num < small) {
                small = num;
            } else if (num < big) {
                big = num;
            } else {
                return true;
            }
        }

        return false;
    }
}
```