Check whether or not an array contains an increasing triplet

triplet_increasing_subarray.cpp

// https://leetcode.com/problems/increasing-triplet-subsequence/
class Solution {
public:
    bool increasingTriplet(vector<int>& nums) {
        if (nums.size() < 3)
            return false;
        int l = 0, r = nums.size() - 1, u = 0;
        // increase l as much as possible
        while (l < nums.size() - 1 && nums[l] >= nums[l + 1])
            ++l;
        if (l >= r - 1)
            return false;
        u = l + 1;
        // reduce r as much as possible
        while (r > 0 && nums[r - 1] >= nums[r])
            --r;
        if (l >= r - 1)
            return false;
        int i = l + 1;
        while (i < r) {
            int cur = nums[i];
            if (cur > nums[u] || (nums[l] < cur && cur < nums[r])) { // u or cur is the middle
                return true;
            } else if (cur < nums[l]) {
                l = i;
            } else if (cur > nums[r] && cur < nums[u]) {
                u = i;
            }
            ++i;
        }
        return false;
    }
};