<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();
        stack<int>s;
        s.push(0);
        // next smaller element
        vector<int>nse(n,n);
        for(int i=1;i<n;i++){
            while(!s.empty() && nums[i]<nums[s.top()]){
                nse[s.top()] = i;
                s.pop();
            }
            s.push(i);
        }
        while(!s.empty()){
            s.pop();
        }
        // previous gretaer
        s.push(n-1);
        vector<int>pge(n,-1);
        for(int i=n-2;i>=0;i--){
            while(!s.empty() && nums[i]>nums[s.top()]){
                pge[s.top()] = i;
                s.pop();
            }
            s.push(i);
        }
        int st = n;
        int end = n-1;
        for(int i=0;i<n;i++){
            if(nse[i]!=n){
                st = i;
                break;
            }
        }
        if(st == n)
        return 0;
        for(int i=n-1;i>=0;i--){
            if(pge[i]!=-1){
                end = i;
                break;
            }
        }
        return end - st + 1;
    }
};
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