<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 result = 0;

        // [1,1,1], 2 = 2
        // [1,2,3]

        // [1,2,3], 3 = 2
        // [1,3,6]

        // [1,2,1,2,1]

        // [1,3,4,6,7], 3 (+1)
        // [0,2,3,5,6], 3 (+1)
        // [0,0,1,3,4], 3 (+1)

        std::vector<int> sums;
        std::inclusive_scan(nums.begin(), nums.end(), std::back_inserter(sums));

        std::vector<int> k_neg;
        k_neg.push_back(k);
        for (int i = 0; i < nums.size(); i++) {
            k_neg.push_back(k + sums[i]);
        }

        /*for (int i = 0; i < nums.size(); i++) {
            for (int j = i; j < nums.size(); j++) {
                if (sums[j] == k_neg[i]) {
                    result += 1;
                }
            }
        }*/

        // sums[j], k_neg[i], j>=i, j !< i

        std::cout << "Sum: ";
        for (auto item: sums) {
            std::cout << item << ",";
        }
        std::cout << std::endl;

        std::cout << "Neg: ";
        for (auto item: k_neg) {
            std::cout << item << ",";
        }
        std::cout << std::endl;

        std::unordered_map<int, int> data;
        for (int i = 0; i < nums.size(); i++) {
            if (data.find(sums[i]) == data.end()) {
                data[sums[i]] = 1;
            } else {
                data[sums[i]] += 1;
            }
        }

        for (int i = 0; i < nums.size(); i++) {
            if (data.find(k_neg[i]) != data.end()) {
                int count = data[k_neg[i]];
                result += count;
            }

            data[sums[i]] -= 1;
        }

        return result;
    }
};
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