<p>Given an integer array <code>nums</code>, you need to find one <b>continuous subarray</b> such that if you only sort this subarray in non-decreasing order, then the whole array will be sorted in non-decreasing order.</p>

<p>Return <em>the shortest such subarray and output its length</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> nums = [2,6,4,8,10,9,15]
<strong>Output:</strong> 5
<strong>Explanation:</strong> You need to sort [6, 4, 8, 10, 9] in ascending order to make the whole array sorted in ascending order.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> nums = [1,2,3,4]
<strong>Output:</strong> 0
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> nums = [1]
<strong>Output:</strong> 0
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= nums.length &lt;= 10<sup>4</sup></code></li>
	<li><code>-10<sup>5</sup> &lt;= nums[i] &lt;= 10<sup>5</sup></code></li>
</ul>

<p>&nbsp;</p>
<strong>Follow up:</strong> Can you solve it in <code>O(n)</code> time complexity?
class Solution {
public:
    int findUnsortedSubarray(vector<int>& nums) {
        int n = nums.size(), i;
        int st = -1, fin = -1;
        stack<int> inc, dec;
        vector<int> pg(n), ns(n);
        // trebuie sa aflu next smaller si previous greater
        for(int i = 0; i < n; i++) {
            while(!dec.empty() && nums[dec.top()] <= nums[i])
                dec.pop();
            if (dec.empty())
                pg[i] = -1;
            else
                pg[i] = dec.top();
            dec.push(i);
        }
        for (int i = n - 1; i >= 0; i--) {
            while(!inc.empty() && nums[inc.top()] >= nums[i])
                inc.pop();
            if (inc.empty())
                ns[i] = -1;
            else
                ns[i] = inc.top();
            inc.push(i);
        }
        for (int i = 0; i < n; i++)
            if (pg[i] != -1 || ns[i] != -1) {
                st = i;
                break;
            }
        for (int i = n - 1; i >= 0; i--)
            if (pg[i] != -1 || ns[i] != -1) {
                fin = i;
                break;
            }
        if (st == -1 && fin == -1)
            return 0;
        return fin - st + 1;

    }
};
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