<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:
    vector<int> previousGreaterLeft(vector<int>& nums){
        int n = nums.size();
        vector<int> pgl(n);

        stack<int> st;

        for(int i=0; i<n; i++){
            while(!st.empty() && nums[st.top()] <= nums[i]) st.pop();
            if(st.empty()) pgl[i] = -1;
            else pgl[i] = st.top();
            st.push(i);
        }

        return pgl;
    }

    vector<int> nextSmallerRight(vector<int>& nums){
        int n = nums.size();
        vector<int> nsr(n);

        stack<int> st;

        for(int i=n-1; i>=0; i--){
            while(!st.empty() && nums[st.top()] >= nums[i]) st.pop();
            if(st.empty()) nsr[i] = -1;
            else nsr[i] = st.top();
            st.push(i);
        }

        return nsr;
    }

    int findUnsortedSubarray(vector<int>& nums) {
        int n = nums.size();
        vector<int> pgl = previousGreaterLeft(nums);
        vector<int> nsr = nextSmallerRight(nums);
        int left = -1;
        int right = -1;
        for(int i=0; i<n; i++){
            if(pgl[i]!=-1 || nsr[i]!=-1){
                left = i;
                break;
            }
        }
        for(int i=n-1; i>=0; i--){
            if(pgl[i]!=-1 || nsr[i]!=-1){
                right = i;
                break;
            }
        }

        if(right==-1 && left==-1) return 0;

        return right-left+1;
    }
};
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