LeetCode-334.Increasing Triplet Subsequence

https://leetcode.com/problems/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.

另x1=x2=max

不断的缩小x1和x2，看看是否有第三个数比x2大.如果出现第三个数 n> x2，则之前必定还有个数x 小于x2，就是说满足 x < x2 < n

bool increasingTriplet(vector<int>& nums)     {        int x1 = 0x7fffffff, x2 = 0x7fffffff;    for (int n : nums)     {    if (n <= x1)     x1 = n;    else if (n <= x2)    x2 = n;    else     return true;    }    return false;    }