一刷400

diguage · Sep 21, 2024 · dd7c814 · dd7c814
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Showing 12 changed files with 81 additions and 9 deletions.
diff --git a/README.adoc b/README.adoc
@@ -2825,14 +2825,14 @@ TIP: **公众号的微信号是: `jikerizhi`**。__因为众所周知的原因
 //|{doc_base_url}/0399-evaluate-division.adoc[题解]
 //|Medium
 //|
-//
-//|{counter:codes}
-//|{leetcode_base_url}/nth-digit/[400. Nth Digit^]
-//|{source_base_url}/_0400_NthDigit.java[Java]
-//|{doc_base_url}/0400-nth-digit.adoc[题解]
-//|Medium
-//|
-//
+
+|{counter:codes}
+|{leetcode_base_url}/nth-digit/[400. Nth Digit^]
+|{source_base_url}/_0400_NthDigit.java[Java]
+|{doc_base_url}/0400-nth-digit.adoc[题解]
+|Medium
+|
+
 //|{counter:codes}
 //|{leetcode_base_url}/binary-watch/[401. Binary Watch^]
 //|{source_base_url}/_0401_BinaryWatch.java[Java]

diff --git a/docs/0400-nth-digit.adoc b/docs/0400-nth-digit.adoc
@@ -35,11 +35,41 @@ _n_ is positive and will fit within the range of a 32-bit signed integer (_n_ <
 The 11th digit of the sequence 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, ... is a 0, which is part of the number 10.
 ----
 
+== 思路分析
 
+image::images/0400-01.png[{image_attr}]
+
+image::images/0400-02.png[{image_attr}]
+
+image::images/0400-03.png[{image_attr}]
+
+image::images/0400-04.png[{image_attr}]
+
+image::images/0400-05.png[{image_attr}]
+
+image::images/0400-06.png[{image_attr}]
+
+image::images/0400-07.png[{image_attr}]
 
 [[src-0400]]
+[tabs]
+====
+一刷::
++
+--
 [{java_src_attr}]
 ----
 include::{sourcedir}/_0400_NthDigit.java[tag=answer]
 ----
+--
+
+// 二刷::
+// +
+// --
+// [{java_src_attr}]
+// ----
+// include::{sourcedir}/_0400_NthDigit_2.java[tag=answer]
+// ----
+// --
+====
 
diff --git a/docs/images/0400-01.png b/docs/images/0400-01.png
diff --git a/docs/images/0400-02.png b/docs/images/0400-02.png
diff --git a/docs/images/0400-03.png b/docs/images/0400-03.png
diff --git a/docs/images/0400-04.png b/docs/images/0400-04.png
diff --git a/docs/images/0400-05.png b/docs/images/0400-05.png
diff --git a/docs/images/0400-06.png b/docs/images/0400-06.png
diff --git a/docs/images/0400-07.png b/docs/images/0400-07.png
diff --git a/docs/index.adoc b/docs/index.adoc
@@ -874,7 +874,7 @@ include::0395-longest-substring-with-at-least-k-repeating-characters.adoc[levelo
 
 // include::0399-evaluate-division.adoc[leveloffset=+1]
 
-// include::0400-nth-digit.adoc[leveloffset=+1]
+include::0400-nth-digit.adoc[leveloffset=+1]
 
 // include::0401-binary-watch.adoc[leveloffset=+1]
 

diff --git a/logbook/202406.adoc b/logbook/202406.adoc
@@ -828,6 +828,12 @@
 |{doc_base_url}/0768-max-chunks-to-make-sorted-ii.adoc[题解]
 |❌ 贪心+单调栈。单调栈的解法思路很精巧！
 
+|{counter:codes}
+|{leetcode_base_url}/nth-digit/[400. Nth Digit^]
+|{doc_base_url}/0400-nth-digit.adoc[题解]
+|❌ 纯纯的数学运算，一脸懵逼
+
+
 |===
 
 截止目前，本轮练习一共完成 {codes} 道题。
diff --git a/src/main/java/com/diguage/algo/leetcode/_0400_NthDigit.java b/src/main/java/com/diguage/algo/leetcode/_0400_NthDigit.java
@@ -0,0 +1,36 @@
+package com.diguage.algo.leetcode;
+
+public class _0400_NthDigit {
+  // tag::answer[]
+
+  /**
+   * @author D瓜哥 · https://www.diguage.com
+   * @since 2024-09-21 18:58:09
+   */
+  public int findNthDigit(int n) {
+    int digit = 1;  // n所在数字有几位数
+    long start = 1; // 每digit位的起始数字，1位数从2开始，2位数从10开始..
+    long count = 9; // 所有digit位数的数位数量, 所有1位数有9个位，所有的2位数90x2个位..
+    // 1. 确定n所在的数字的位数digit
+    //  循环执行 n 减去 一位数、两位数、... 的数位数量 count ，直至 n≤count 时跳出。
+    while (n > count) {
+      n -= count;  // n分别减去1位数 2位数 3位数的个数..
+      start *= 10;
+      digit += 1;
+      count = digit * start * 9;
+    }
+    // 2. 确定n所在的数字num，n-1是从0开始计算偏移量
+    //   所求数位在从数字 start 开始的第 [(n−1)/digit] 个 数字 中（start 为第 0 个数字）。
+    long num = start + (n - 1) / digit;
+    // 3. 确定所求数位是 num 的第几位数字，从0算起
+    //    所求数位为数字 num 的第 (n−1)%digit 位（数字的首个数位为第 0 位）。
+    // 注：上面第二步已经确定在哪个数字了，那么取余就是找在该数中的第多少位。
+    //     能整除的部分，已经在前面n-1个数字中了
+    return Long.toString(num).charAt((n - 1) % digit) - '0';
+  }
+  // end::answer[]
+
+  public static void main(String[] args) {
+    new _0400_NthDigit().findNthDigit(300);
+  }
+}