> For the complete documentation index, see [llms.txt](https://aaronice.gitbook.io/lintcode/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://aaronice.gitbook.io/lintcode/string.md).

# String

> A string is actually an array of `unicode characters`.

## String Comparison

String has its own `compare function`

> Can we use "==" to compare two strings?

Depends on

> Does the language support `operator overloading`?

Yes (like C++), we may use "==" to compare strings

No (like Java), we may not use "==" to compare two strings. "==" actually compares whether these two objects are the same object.

For Java, use

```
s1.equals(s2)
```

or

```
s1.compareTo(s2) == 0
```

## Immutable or Mutable

In some languages (like C++), string is `mutable`, while in Java, string is `immutable`

**Time Complexity of Built-in Operations**

For instance, if the length of the string is`N`, the time complexity of both finding operation and substring operation is`O(N)`.

```java
// "static void main" must be defined in a public class.
public class Main {
    public static void main(String[] args) {
        String s1 = "Hello World";

        // 1. concatenate
        s1 += "!";
        System.out.println(s1);

        // 2. find
        System.out.println("The position of first 'o' is: " + s1.indexOf('o'));
        System.out.println("The position of last 'o' is: " + s1.lastIndexOf('o'));

        // 3. get substring
        System.out.println(s1.substring(6, 11));
    }
}
```

### Beware of String Concatenation in Java

> In Java, since the string is `immutable`, concatenation works by first allocating enough space for the new string, copy the contents from the old string and append to the new string.

In Java, using "+" for string concatenation, the **time complexity will be O(n)**

For the following, total **time complexity in total will be O(n^2)**

```java
// "static void main" must be defined in a public class.
public class Main {
    public static void main(String[] args) {
        String s = "";
        int n = 10000;
        for (int i = 0; i < n; i++) {
            s += "hello";
        }
    }
}
```

## Mutate String in Java

### 1. convert it to a char array

```java
// "static void main" must be defined in a public class.
public class Main {
    public static void main(String[] args) {
        String s = "Hello World";
        char[] str = s.toCharArray();
        str[5] = ',';
        System.out.println(str);
    }
}
```

### 2. **use some other data structures like** `StringBuilder`

The below code runs in **O(n) time complexity.**

```java
// "static void main" must be defined in a public class.
public class Main {
    public static void main(String[] args) {
        int n = 10000;
        StringBuilder str = new StringBuilder();
        for (int i = 0; i < n; i++) {
            str.append("hello");
        }
        String s = str.toString();
    }
}
```


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://aaronice.gitbook.io/lintcode/string.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.