<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:
    vector<int> composePrefixSumArr(int n, vector<int>nums) {
        vector<int> prefixSumArr({nums[0]});
        for(int i = 1; i < n; i++) {
            prefixSumArr.push_back(nums[i] + prefixSumArr[i-1]);
        }
        return prefixSumArr;
    }

    int subarraySum(vector<int>& nums, int k) {
        int n = nums.size();
        vector<int>prefixSumArr = composePrefixSumArr(n, nums);
        int countOfSubArrSumEqualToK = 0;
        int initialVal = 0;
        for(int i = 0; i < n; i++) {
            for(int j = i; j < n; j++) {
                if(prefixSumArr[j] - initialVal == k) countOfSubArrSumEqualToK++;
            }
            initialVal = prefixSumArr[i];
        }

        return countOfSubArrSumEqualToK;
    }
};
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