Create TwySum.md

TwoSum/TwoSum.md

@@ -0,0 +1,96 @@
+# Step 1
+まずは愚直に実行
+
+```python
+
+class Solution(object):
+    def twoSum(self, nums, target):
+        """
+        :type nums: List[int]
+        :type target: int
+        :rtype: List[int]
+        """
+        for i in range(len(nums)):
+            for j in range(i+1, len(nums)):
+                if nums[i]+nums[j]==target:
+                    return [i,j]
+                    break
+```
+実行時間は2271ms
+
+
+
+# Step 2
+PRや公式解答を参照．
+解答を見るとhashmapという手法を発見．dictを用いることで高速に探索出来るらしい．
+keyの操作に慣れていないので，デバッグしつつ実装
+```python
+class Solution(object):
+    def twoSum(self, nums, target):
+        """
+        :type nums: List[int]
+        :type target: int
+        :rtype: List[int]
+        """
+        hashmap = {}
+        for i in range(len(nums)):
+            hashmap[nums[i]] = i
+        print(nums) # [2, 7, 11, 15]
+        print(hashmap) # {15: 3, 2: 0, 11: 2, 7: 1}
+        for i in range(len(nums)):
+            complement = target - nums[i]
+            if complement in hashmap and hashmap[complement] != i: # dict[key]=value
+                return [i, hashmap[complement]]
+        return []
+```
+実行時間は158ms．計算量を減らすことが出来た．
+
+# Step 3
+何も見ないで実装
+```python
+class Solution(object):
+    def twoSum(self, nums, target):
+        """
+        :type nums: List[int]
+        :type target: int
+        :rtype: List[int]
+        """
+        hashmap = {}
+        for i in range(len(nums)):
+            hashmap[nums[i]] = i
+        for i in range(len(nums)):
+            complement = target - nums[i]
+            if complement in hashmap and hashmap[complement]!=i:
+                return [i, hashmap[complement]]
+        return []
+```
+実行時間は5ms同じコードなのに実行時間が違う．他に見るべき指標があるかも．
+
+# Step 4
+いただいたアドバイスを元に実装
+```python
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        num_to_index = {}
+        for i in range (len(nums)):
+            complement = target - nums[i]
+            if complement in num_to_index and num_to_index[complement]!=i:
+                return [i, num_to_index[complement]]
+            num_to_index[nums[i]] = i
+        return []
+```
+辞書の名前をnum_to_indexに変更して意味を分かりやすくした．
+for文を1つにまとめた．
+
+```python
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        num_to_index = {}
+        for i, num in enumerate(nums):
+            complement = target - num
+            if complement in num_to_index and num_to_index[complement]!=i:
+                return [num_to_index[complement], i]
+            num_to_index[num] = i
+        return []
+```
+enumerateでも書いてみた．個人的にはこちらのほうが好みかも．