Create subarray-sum-equals-k.md

subarray-sum-equals-k.md

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+自分で書いてTLEしたもの。
+
+```py
+class Solution:
+    def subarraySum(self, nums: List[int], k: int) -> int:
+        res = 0
+        for index, num in enumerate(nums):
+            sum = 0
+            j = 0
+            while index + j <= len(nums) - 1:
+                sum += nums[index + j]
+                j += 1
+                if sum == k:
+                    res += 1
+        return res
+```
+
+1回目：解答を見てなんとかアクセプト。
+
+```py
+from collections import defaultdict
+class Solution:
+    def subarraySum(self, nums: List[int], k: int) -> int:
+        res = 0
+        sum_to_counts = defaultdict(int)
+        sum_to_counts[0] = 1
+        current_sum = 0
+        for num in nums:
+            current_sum += num
+            if current_sum - k in sum_to_counts:
+                res += sum_to_counts[current_sum - k]
+            sum_to_counts[current_sum] += 1
+        return res
+```
+
+2回目：renameとif節の削除。
+
+```py
+from collections import defaultdict
+class Solution:
+    def subarraySum(self, nums: List[int], k: int) -> int:
+        res = 0
+        current_sum = 0
+        sum_count = defaultdict(int)
+        sum_count[0] = 1
+        for num in nums:
+            current_sum += num
+            res += sum_count[current_sum - k]
+            sum_count[current_sum] += 1
+        return res
+```
+
+3回目：current -> cur とrename.
+
+```py
+from collections import defaultdict
+class Solution:
+    def subarraySum(self, nums: List[int], k: int) -> int:
+        res = 0
+        cur_sum = 0
+        sum_count = defaultdict(int)
+        sum_count[0] = 1
+        for num in nums:
+            cur_sum += num
+            res += sum_count[cur_sum - k]
+            sum_count[cur_sum] += 1
+        return res
+```