<p>Given an integer array <code>nums</code> and an integer <code>k</code>, return <em>the length of the shortest non-empty <strong>subarray</strong> of </em><code>nums</code><em> with a sum of at least </em><code>k</code>. If there is no such <strong>subarray</strong>, return <code>-1</code>.</p>

<p>A <strong>subarray</strong> is a <strong>contiguous</strong> part of an array.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<pre><strong>Input:</strong> nums = [1], k = 1
<strong>Output:</strong> 1
</pre><p><strong class="example">Example 2:</strong></p>
<pre><strong>Input:</strong> nums = [1,2], k = 4
<strong>Output:</strong> -1
</pre><p><strong class="example">Example 3:</strong></p>
<pre><strong>Input:</strong> nums = [2,-1,2], k = 3
<strong>Output:</strong> 3
</pre>
<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= nums.length &lt;= 10<sup>5</sup></code></li>
	<li><code>-10<sup>5</sup> &lt;= nums[i] &lt;= 10<sup>5</sup></code></li>
	<li><code>1 &lt;= k &lt;= 10<sup>9</sup></code></li>
</ul>

class Solution {
public:
    int shortestSubarray(vector<int>& nums, int k) {
        vector<long long> pref;
        long long sum = 0;
        for(auto i: nums){
            sum += i;
            pref.push_back(sum);
        }
        vector<int> st;
        int ans = 1e9;
        for(int i=0;i<pref.size();i++){
            int start = 0, en = st.size()-1, ind = -1;
            while(start<=en){
                int m = (start+en)/2;
                if((pref[st[m]]+k) <= pref[i]){
                    start = m+1;
                    ind = st[m];
                }else{
                    en = m-1;
                }
            }
            if(ind!=-1){
                ans = min((int)ans, (int)i-ind);
            }
            if(pref[i]>=k){
                ans = min(ans, i+1);
            }
            while(st.size() && (pref[st.back()]+k) >= (pref[i]+k)){
                st.pop_back();
            }     
            st.push_back(i);
        }
        return ((ans!=1e9)?ans: -1);
    }
};
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