2019-11-02 12:07:41 +08:00
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# 44. 数字序列中的某一位数字
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## 题目描述
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数字以 0123456789101112131415... 的格式序列化到一个字符串中，求这个字符串的第 index 位。
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## 解题思路
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```java
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public int getDigitAtIndex(int index) {
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if (index < 0)
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return -1;
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int place = 1; // 1 表示个位，2 表示 十位...
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while (true) {
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int amount = getAmountOfPlace(place);
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int totalAmount = amount * place;
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if (index < totalAmount)
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return getDigitAtIndex(index, place);
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index -= totalAmount;
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place++;
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}
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}
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/**
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* place 位数的数字组成的字符串长度
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* 10, 90, 900, ...
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*/
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private int getAmountOfPlace(int place) {
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if (place == 1)
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return 10;
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return (int) Math.pow(10, place - 1) * 9;
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}
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/**
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* place 位数的起始数字
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* 0, 10, 100, ...
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*/
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private int getBeginNumberOfPlace(int place) {
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if (place == 1)
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return 0;
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return (int) Math.pow(10, place - 1);
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}
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/**
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* 在 place 位数组成的字符串中，第 index 个数
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*/
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private int getDigitAtIndex(int index, int place) {
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int beginNumber = getBeginNumberOfPlace(place);
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int shiftNumber = index / place;
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String number = (beginNumber + shiftNumber) + "";
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int count = index % place;
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return number.charAt(count) - '0';
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}
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```
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2019-11-02 17:33:10 +08:00
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<div align="center"><img width="320px" src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/githubio/公众号二维码-2.png"></img></div>
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