Add new problem

Signed-off-by: begeekmyfriend <begeekmyfriend@gmail.com>

Author: begeekmyfriend

0560_subarray_sum_equals_k/subarray_sum.cc

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+#include <bits/stdc++.h>
+
+using namespace std;
+
+class Solution {
+public:
+    int subarraySum(vector<int>& nums, int k) {
+        int res = 0, sum = 0;
+        unordered_map<int, int> pre_sum_cnt;
+
+        // The prefix sum array records the sum of nums[0...i], so we have
+        // presum[j] - presum[j] = k when the sum of nums[i...j] equals k.
+        // The presum[0] should always be 0. And pre_sum_cnt[0] = 1.
+        pre_sum_cnt[0] = 1;
+        for (const auto n : nums) {
+            // Here the sum means sum of nums[0...j] and the sum0 means sum
+            // of nums[0...i] then there will be sum - sum0 = k.
+            sum += n;
+            int sum0 = sum - k;
+            if (ht.count(sum0)) {
+                res += pre_sum_cnt[sum0];
+            }
+            pre_sum_cnt[sum]++;
+        }
+
+        return res;
+    }
+};