leetcode 7. Reverse Integer

Author: myoshi2891

Algorithm/Other/leetcode/7. Reverse Integer/README.md

@@ -0,0 +1,115 @@
+この解法は、64ビット整数を使わずに、32ビット整数の範囲を超えないかを都度チェックしながら桁を逆にしていきます。
+
+---
+
+### ✅ TypeScriptコード（`fs`での入力読み取り対応）
+
+```ts
+import * as fs from 'fs';
+
+const input = fs.readFileSync('/dev/stdin', 'utf8').trim();
+const x = parseInt(input);
+
+/**
+ * 32ビット整数の範囲
+ */
+const INT_MIN = -(2 ** 31);
+const INT_MAX = 2 ** 31 - 1;
+
+function reverse(x: number): number {
+    let result = 0;
+    let num = x;
+
+    while (num !== 0) {
+        const digit = num % 10 | 0; // 小数点対策で |0 して整数化
+        num = (num / 10) | 0;
+
+        // resultが次の桁を追加したときにオーバーフローしないか確認
+        if (
+            result > Math.floor(INT_MAX / 10) ||
+            (result === Math.floor(INT_MAX / 10) && digit > 7)
+        ) {
+            return 0;
+        }
+        if (
+            result < Math.ceil(INT_MIN / 10) ||
+            (result === Math.ceil(INT_MIN / 10) && digit < -8)
+        ) {
+            return 0;
+        }
+
+        result = result * 10 + digit;
+    }
+
+    return result;
+}
+
+console.log(reverse(x));
+```
+
+---
+
+### 🔍 処理概要（図付きで説明）
+
+たとえば `x = -123` のとき：
+
+```
+初期状態: result = 0, num = -123
+
+ループ1:
+    digit = -123 % 10 = -3
+    num = -123 / 10 = -12
+    result = 0 * 10 + (-3) = -3
+
+ループ2:
+    digit = -12 % 10 = -2
+    num = -12 / 10 = -1
+    result = -3 * 10 + (-2) = -32
+
+ループ3:
+    digit = -1 % 10 = -1
+    num = -1 / 10 = 0
+    result = -32 * 10 + (-1) = -321
+
+終了（num === 0）→ return -321
+```
+
+---
+
+### 🛑 オーバーフローの例
+
+例：`x = 1534236469`
+
+```
+途中で result = 964632435 になり、
+次の桁（9）を追加すると 9646324350 + 9 = 9646324359 > INT_MAX
+→ return 0
+```
+
+---
+
+### ✅ テストケース例
+
+```plaintext
+Input:
+123
+Output:
+321
+
+Input:
+-123
+Output:
+-321
+
+Input:
+120
+Output:
+21
+
+Input:
+1534236469
+Output:
+0
+```
+
+---

Algorithm/Other/leetcode/7. Reverse Integer/ReverseInteger.ts

@@ -0,0 +1,40 @@
+import * as fs from 'fs';
+
+const input = fs.readFileSync('/dev/stdin', 'utf8').trim();
+const x = parseInt(input);
+
+/**
+ * 32ビット整数の範囲
+ */
+const INT_MIN = -(2 ** 31);
+const INT_MAX = 2 ** 31 - 1;
+
+function reverse(x: number): number {
+    let result = 0;
+    let num = x;
+
+    while (num !== 0) {
+        const digit = num % 10 | 0; // 小数点対策で |0 して整数化
+        num = (num / 10) | 0;
+
+        // resultが次の桁を追加したときにオーバーフローしないか確認
+        if (
+            result > Math.floor(INT_MAX / 10) ||
+            (result === Math.floor(INT_MAX / 10) && digit > 7)
+        ) {
+            return 0;
+        }
+        if (
+            result < Math.ceil(INT_MIN / 10) ||
+            (result === Math.ceil(INT_MIN / 10) && digit < -8)
+        ) {
+            return 0;
+        }
+
+        result = result * 10 + digit;
+    }
+
+    return result;
+}
+
+console.log(reverse(x));

DataStructures/Trees/BinaryIndexedTree/FenwickTree/atcoder/A76/README.md

@@ -127,3 +127,11 @@ dp:        1   0   1    2    2    1    7
 | 最終出力              | `dp[ゴールのインデックス]` が答え            |
 
 ---
+
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+| 2025-07-01 18:36:39 | [A76 - River Crossing](https://atcoder.jp/contests/tessoku-book/tasks/tessoku_book_bx) | [myoshizumi](https://atcoder.jp/users/myoshizumi) | [PHP (php 8.2.8)](https://atcoder.jp/contests/tessoku-book/submissions/me?f.Language=5016) | 1000 | 2451 Byte |  | 248 ms | 46648 KiB | [詳細](https://atcoder.jp/contests/tessoku-book/submissions/67219001) |
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+| 2025-07-01 18:27:47 | [A76 - River Crossing](https://atcoder.jp/contests/tessoku-book/tasks/tessoku_book_bx) | [myoshizumi](https://atcoder.jp/users/myoshizumi) | [Python (CPython 3.11.4)](https://atcoder.jp/contests/tessoku-book/submissions/me?f.Language=5055) | 1000 | 1741 Byte |  | 473 ms | 39756 KiB | [詳細](https://atcoder.jp/contests/tessoku-book/submissions/67218847) |
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