leetcode-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.

 1 class Solution {
 2 public:
 3     bool increasingTriplet(vector<int>& nums) {
 4         int m1 = 0x7fffffff, m2 = 0x7fffffff;
 5         for (int a : nums) {
 6             if (m1 >= a) m1 = a;
 7             else if (m2 >= a) m2 = a;
 8             else return true;
 9         }
10         return false;
11     }
12 };