<p>Given an array of integers <code>nums</code> and an integer <code>k</code>, return <em>the total number of subarrays whose sum equals to</em> <code>k</code>.</p>

<p>A subarray is a contiguous <strong>non-empty</strong> sequence of elements within an array.</p>

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

<ul>
	<li><code>1 &lt;= nums.length &lt;= 2 * 10<sup>4</sup></code></li>
	<li><code>-1000 &lt;= nums[i] &lt;= 1000</code></li>
	<li><code>-10<sup>7</sup> &lt;= k &lt;= 10<sup>7</sup></code></li>
</ul>

class Solution {
public:
    int subarraySum(vector<int>& nums, int k) 
    {
        int n=nums.size();
        vector<int> presum(n+1);
        presum.push_back(0);
        presum.push_back(nums[0]);

        for(int i = 1;i<n+1;i++)
        {
            presum[i] = presum[i-1] +nums[i-1]; 
        }

        int cnt = 0;

        for(int i = 0;i<n;i++)
        {
            for(int j=i+1;j<n+1;j++)
            {
                if(presum[j]-presum[i] == k)
                    cnt++;
            }
        }    
        return cnt;
    }
};
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