From e7d3dcd241e994ba63a27ab6a8bc541df4195b58 Mon Sep 17 00:00:00 2001
From: Kazuki Kitano <kitano-kazuki954@g.ecc.u-tokyo.ac.jp>
Date: Tue, 2 Jun 2026 14:47:31 +0900
Subject: [PATCH] step123

---
 memo.md | 96 +++++++++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 93 insertions(+), 3 deletions(-)

diff --git a/memo.md b/memo.md
index efb0777..6fa0765 100644
--- a/memo.md
+++ b/memo.md
@@ -2,12 +2,102 @@
 
 ## アプローチ
 
-## Code1-1
+* 二進数の足し算を行う
+* 各位について, carry, a, bの３つを受け取って, carryを次の位に引き継ぎつつ、結果にcarry以外の部分を加える
+* 1:28
+
+## Code1-1 (while)
+
+* AC: 6:41
+* `a_i`や`b_i`が使わなくなっても引かれ続けているのは若干きになる
+
+```python
+class Solution:
+    def addBinary(self, a: str, b: str) -> str:
+        sum_binary = []
+        carry = 0
+        a_i = len(a) - 1
+        b_i = len(b) - 1
+        while 0 <= a_i or 0 <= b_i or carry == 1:
+            a_bit = int(a[a_i]) if 0 <= a_i else 0
+            b_bit = int(b[b_i]) if 0 <= b_i else 0
+
+            sum_bits = a_bit + b_bit + carry
+
+            sum_binary.append(str(sum_bits % 2))
+            carry = sum_bits // 2
+            a_i -= 1
+            b_i -= 1
+        
+        return "".join(list(reversed(sum_binary)))
+
+```
 
 # Step2
 
-## Code2-1
+## Code2-1 (while)
+
+* 変数の命名を変えた
+* `summed_bits`と`sum_bits`が混在しているのはわかりにくいけど、ほかにいい名前思いつかず
+
+```python
+class Solution:
+    def addBinary(self, a: str, b: str) -> str:
+        summed_bits = []
+        carry = 0
+        i_a = len(a) - 1
+        i_b = len(b) - 1
+        while 0 <= i_a or 0 <= i_b or carry == 1:
+            a_bit = int(a[i_a]) if 0 <= i_a else 0
+            b_bit = int(b[i_b]) if 0 <= i_b else 0
+
+            sum_bits = a_bit + b_bit + carry
+
+            summed_bits.append(str(sum_bits % 2))
+            carry = sum_bits // 2
+            if 0 <= i_a:
+                i_a -= 1
+            if 0 <= i_b:
+                i_b -= 1
+
+        
+        return "".join(list(reversed(summed_bits)))
+
+```
+
+## 他の人のPRを見る
+
+* https://github.com/ryosuketc/leetcode_grind75/pull/20
+* https://github.com/huyfififi/coding-challenges/pull/20
+    * `zip_longest`を使うとシンプルで良さそう
+    * ほかにも, 最初に`a`や`b`をpaddingする方法がある
+        * `"0" * (len(a) - len(b))`としたときに, `len(a) - len(b)`がマイナスだと結果は`""`になる
 
 # Step3
 
-## Code3-1
+## Code3-2 (zip longest)
+
+* 1:52
+* 1:24
+* 1:04
+
+```python
+import itertools
+import collections
+
+
+class Solution:
+    def addBinary(self, a: str, b: str) -> str:
+        summed_bits = collections.deque([])
+        carry = 0
+        for bit_a, bit_b in itertools.zip_longest(reversed(a), reversed(b), fillvalue="0"):
+            total = int(bit_a) + int(bit_b) + carry
+            summed_bits.appendleft(str(total % 2))
+            carry = total // 2
+        
+        if carry == 1:
+            summed_bits.appendleft("1")
+        
+        return "".join(summed_bits)
+
+```
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