> For the complete documentation index, see [llms.txt](https://liuyang89116.gitbook.io/my-leetcode-book/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://liuyang89116.gitbook.io/my-leetcode-book/chapter_2_string/problem_383_ransom_note.md).

# Problem 383: Ransom Note

> <https://leetcode.com/problems/ransom-note/>

## 思路

* 就是一个变相的哈希表的计数
* 这道题虽然简单，但是涉及到的东西还挺多的

```java
public class Solution {
    public boolean canConstruct(String ransomNote, String magazine) {
       int[] cnt = new int[26];
       for (int i = 0; i < magazine.length(); i++) {
           cnt[magazine.charAt(i) - 97]++;
       }
       for (int i = 0; i < ransomNote.length(); i++) {
           if(--cnt[ransomNote.charAt(i) - 97] < 0) {
               return false;
           }
       }

       return true;    
    }
}
```

## 易错点

1. 字母的 ASCII 码

   a - z: 97 - 122

   A - Z: 65 - 90

   0 - 9: 48 - 57
2. 几个非常容易错的知识点

   ```java
   String s  = "abc";
   System.out.println(s.charAt(0));          // a   
   System.out.println(s.charAt(0) - '0');    // 49
   System.out.println(s.charAt(0) - 0);      // 97
   ```

   ```java
   s.charAt(0) - '0'
   ```

   这里相当于是`'a' - '0'`，两个 char 相减，减的是他们的 ASCII 值：$$97 - 48 = 49$$

   ```java
   s.charAt(0) - 0
   ```

   这里相当于是 char 和 int 相减，$$97 - 0 = 97$$