Maximum Length of Semi-Decreasing Subarrays — C++

// Time:  O(n)
// Space: O(n)

// mono stack
class Solution {
public:
    int maxSubarrayLength(vector<int>& nums) {
        vector<int> stk;
        for (int i = size(nums) - 1; i >= 0; --i) {
            if (empty(stk) || nums[stk.back()] > nums[i]) {
                stk.emplace_back(i);
            }
        }
        int result = 0;
        for (int left = 0; left < size(nums); ++left) {
            while (!empty(stk) && nums[stk.back()] < nums[left]) {
                result = max(result, stk.back() - left + 1); stk.pop_back();
            }
        }
        return result;
    }
};

// Time:  O(nlogn)
// Space: O(n)
// sort
class Solution2 {
public:
    int maxSubarrayLength(vector<int>& nums) {
        vector<int> idxs(size(nums));
        iota(begin(idxs), end(idxs), 0);
        sort(rbegin(idxs), rend(idxs), [&](const auto& a, const auto& b) { return nums[a] < nums[b]; });
        int result = 0;
        for (int left = 0; left < size(nums); ++left) {
            while (!empty(idxs) && nums[idxs.back()] < nums[left]) {
                result = max(result, idxs.back() - left + 1); idxs.pop_back();
            }
        }
        return result;
    }
};