Add 1. Two Sum.md

1. Two Sum.md

@@ -0,0 +1,93 @@
+https://leetcode.com/problems/two-sum/
+
+### Description
+
+Given an array of integers `nums` and an integer `target`, return _indices of the two numbers such that they add up to `target`_.
+
+You may assume that each input would have **_exactly_ one solution**, and you may not use the _same_ element twice.
+
+You can return the answer in any order.
+
+**Example 1:**
+
+**Input:** nums = [2,7,11,15], target = 9
+**Output:** [0,1]
+**Explanation:** Because nums[0] + nums[1] == 9, we return [0, 1].
+
+**Example 2:**
+
+**Input:** nums = [3,2,4], target = 6
+**Output:** [1,2]
+
+**Example 3:**
+
+**Input:** nums = [3,3], target = 6
+**Output:** [0,1]
+
+**Constraints:**
+
+- `2 <= nums.length <= 104`
+- `-109 <= nums[i] <= 109`
+- `-109 <= target <= 109`
+- **Only one valid answer exists.**
+
+**Follow-up:** Can you come up with an algorithm that is less than `O(n2)` time complexity?
+### STEP1
+brute forceしか思いつかなかったので解答をみた。
+nをnumsの長さとして
+Time ComplexityO(n) Space ComplexityO(n)
+```python
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        d = {}
+        for i, x in enumerate(nums):
+            d[x] = i
+        for i, x in enumerate(nums):
+            if target - x in d and i != d[target - x]:
+                return [i,d[target-x]]
+```
+
+### STEP2
+あまりに辞書の変数が酷いけどどうしたもんかなと思っていたら
+https://github.com/ksaito0629/leetcode_arai60/pull/10/changes/bb1ae591a98e7810711de49f212bb97f5fc34ce9#r2811748134
+辞書の名前としてkey_to_valueはなるほど感がある。
+というか2周ループを回す必要は別になくて探しながら追加したらいいだけだな。
+あとleetcodeで書くときにエラーをraiseする習慣はなかったけど他の人のコードを見ていると書いているっぽい。仕事なら書いてるけども。
+
+```python
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        num_to_index = {}
+        for index, num in enumerate(nums):
+            if target - num in num_to_index:
+                return [index, num_to_index[target-num]]
+            num_to_index[num] = index
+        raise NoValueError("There is no such pair that sums to the target")
+```
+brute forceも一応書いておく。
+Time ComplexityO(n^2) Space ComplexityO(1)
+```python
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        length = len(nums)
+        for i in range(length):
+            for j in range(i+1, length):
+                if nums[i] + nums[j] == target:
+                    return [i,j]
+        raise NoValueError("There is no such pair that sums to the target.")
+```
+
+### STEP3
+```python
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        num_to_index = {}
+        for index, num in enumerate(nums):
+            if target - num in num_to_index:
+                return [index, num_to_index[target-num]]
+            num_to_index[num] = index
+        raise NoValueError("There is no such pair that sums to the target")
+```
+
+そういえばpythonのエラーってどんな感じだったっけと思って見直した
+https://docs.python.org/3.15/tutorial/errors.html