From dc104a16169dc532a682098dbb0181be90a87e0d Mon Sep 17 00:00:00 2001
From: akmhmgc <akimuhamaguchi@icloud.com>
Date: Thu, 25 Sep 2025 19:39:34 +0900
Subject: [PATCH] 122

---
 122/step1.md | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 122/step2.md | 19 +++++++++++++++++++
 122/step3.md | 14 ++++++++++++++
 122/step4.md |  1 +
 4 files changed, 82 insertions(+)
 create mode 100644 122/step1.md
 create mode 100644 122/step2.md
 create mode 100644 122/step3.md
 create mode 100644 122/step4.md

diff --git a/122/step1.md b/122/step1.md
new file mode 100644
index 0000000..71188b9
--- /dev/null
+++ b/122/step1.md
@@ -0,0 +1,48 @@
+# step1 何も見ずに解く
+
+簡単な例で確認した。
+ある時点を考えると、一つ前の値段が安い時はその時に買って、ある時点で売れば良いだけであることに気づいた。
+時間計算量はO(N)で空間計算量はO(1)
+
+```ruby
+# @param {Integer[]} prices
+# @return {Integer}
+def max_profit(prices)
+    prices_size = prices.size
+    return 0 if prices_size <= 1
+
+    max_profit = 0
+    buy_price = prices.first
+    prices.each do |sell_price|
+        if sell_price > buy_price
+            max_profit += sell_price - buy_price
+        end
+        buy_price = sell_price
+    end
+    max_profit
+end
+```
+
+もう少し見やすくできる。
+
+```ruby
+# @param {Integer[]} prices
+# @return {Integer}
+def max_profit(prices)
+    max_profit = 0
+    prices.each_cons(2) do |prev_price, current_price|
+        next unless current_price > prev_price
+        max_profit += current_price - prev_price
+    end
+    max_profit
+end
+```
+
+これもRubyっぽくてありかもしれない。
+```ruby
+# @param {Integer[]} prices
+# @return {Integer}
+def max_profit(prices)
+    prices.each_cons(2).map { |prev_price, current_price| [0, current_price - prev_price].max }.inject(:+) || 0
+end
+```
diff --git a/122/step2.md b/122/step2.md
new file mode 100644
index 0000000..044921d
--- /dev/null
+++ b/122/step2.md
@@ -0,0 +1,19 @@
+# step2 他の方の解答を見る
+## 最小売買回数
+https://github.com/Yoshiki-Iwasa/Arai60/pull/53#discussion_r1730194725
+
+up trendが終わったタイミングで売買を行えば良いので、以下のような感じで出せる。
+
+```ruby
+# @param {Integer[]} prices
+# @return {Integer}
+def min_trading_count(prices)
+    min_trading_count = 0
+    is_up_trend = false
+    prices.each_cons(2) do |prev_price, current_price|
+        min_trading_count += 1 if is_up_trend && current_price < prev_price
+        is_up_trend = current_price > prev_price
+    end
+    min_trading_count
+end
+```
diff --git a/122/step3.md b/122/step3.md
new file mode 100644
index 0000000..40bd939
--- /dev/null
+++ b/122/step3.md
@@ -0,0 +1,14 @@
+# step3 3回続けて10分以内に書いてエラーを出さなければOKとする
+
+```ruby
+# @param {Integer[]} prices
+# @return {Integer}
+def max_profit(prices)
+    max_profit = 0
+    prices.each_cons(2) do |prev_price, current_price|
+        next unless current_price > prev_price
+        max_profit += current_price - prev_price
+    end
+    max_profit
+end
+```
diff --git a/122/step4.md b/122/step4.md
new file mode 100644
index 0000000..5941ee1
--- /dev/null
+++ b/122/step4.md
@@ -0,0 +1 @@
+## step4 レビューを受けて解答を修正