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.

思路：先在左边找一个最小值l(如果遇到小的就更新)，如果当前值没用来更新l那么试着更新m，如果没更新m那么就存在一个符合条件的Triplet

class Solution {
public:
    bool increasingTriplet(vector<int>& nums) {
        if (nums.size() < 3) return false;
        int l = nums[0];
        int m = INT_MAX;
        for (int i = 1; i < nums.size(); ++i) {
            if (nums[i] <= l) l = nums[i];
            else if (nums[i] < m) m = nums[i];
            else if (nums[i] > m) return true;
        }
        return false;
    }
};