leetCode学习-探索数组与字符串
数组简介
数组简介
- 数组是一种基本的数据结构,用于按顺序存储元素的集合。但是元素可以随机存取,因为数组中的每个元素都可以通过数组索引来识别。
- Operate in an array
public class Main {
public static void main(String[] args) {
// 1. Initialize
int[] a0 = new int[5];
int[] a1 = {1, 2, 3};
// 2. Get Length
System.out.println("The size of a1 is: " + a1.length);
// 3. Access Element
System.out.println("The first element is: " + a1[0]);
// 4. Iterate all Elements
System.out.print("[Version 1] The contents of a1 are:");
for (int i = 0; i < a1.length; ++i) {
System.out.print(" " + a1[i]);
}
System.out.println();
System.out.print("[Version 2] The contents of a1 are:");
for (int item: a1) {
System.out.print(" " + item);
}
System.out.println();
// 5. Modify Element
a1[0] = 4;
// 6. Sort
Arrays.sort(a1);
}
}
动态数组简介
大多数编程语言都提供内置的动态数组,它仍然是一个随机存取的列表数据结构,但大小是可变的。例如,在 C++ 中的 vector,以及在 Java 中的 ArrayList。
public class Main {
public static void main(String[] args) {
// 1. initialize
List<Integer> v0 = new ArrayList<>();
List<Integer> v1; // v1 == null
// 2. cast an array to a vector
Integer[] a = {0, 1, 2, 3, 4};
v1 = new ArrayList<>(Arrays.asList(a));
// 3. make a copy
List<Integer> v2 = v1; // another reference to v1
List<Integer> v3 = new ArrayList<>(v1); // make an actual copy of v1
// 3. get length
System.out.println("The size of v1 is: " + v1.size());;
// 4. access element
System.out.println("The first element in v1 is: " + v1.get(0));
// 5. iterate the vector
System.out.print("[Version 1] The contents of v1 are:");
for (int i = 0; i < v1.size(); ++i) {
System.out.print(" " + v1.get(i));
}
System.out.println();
System.out.print("[Version 2] The contents of v1 are:");
for (int item : v1) {
System.out.print(" " + item);
}
System.out.println();
// 6. modify element
v2.set(0, 5); // modify v2 will actually modify v1
System.out.println("The first element in v1 is: " + v1.get(0));
v3.set(0, -1);
System.out.println("The first element in v1 is: " + v1.get(0));
// 7. sort
Collections.sort(v1);
// 8. add new element at the end of the vector
v1.add(-1);
v1.add(1, 6);
// 9. delete the last element
v1.remove(v1.size() - 1);
}
}
二维数组简介
类似于一维数组,二维数组也是由元素的序列组成。但是这些元素可以排列在矩形网格中而不是直线上。
二维数组简介
- C++ 将二维数组存储为一维数组。
下图显示了大小为 M * N 的数组 A 的实际结构:
- 在Java中,二维数组实际上是包含着 M 个元素的一维数组,每个元素都是包含有 N 个整数的数组。
下图显示了 Java 中二维数组 A 的实际结构:
字符串简介
字符串简介
字符串实际上是一个 unicode 字符数组
- 比较函数:
- == 比较地址的值
- equals() String的equals()比较内容,Object的equals()类似==,比较地址的值
- compareTo() 比较对应字符的大小(ASCII码顺序)
- 代码:
public class Main {
public static void main(String[] args) {
// initialize
String s1 = "Hello World";
System.out.println("s1 is \"" + s1 + "\"");
String s2 = s1;
System.out.println("s2 is another reference to s1.");
String s3 = new String(s1);
System.out.println("s3 is a copy of s1.");
// compare using '=='
System.out.println("Compared by '==':");
// true since string is immutable and s1 is binded to "Hello World"
System.out.println("s1 and \"Hello World\": " + (s1 == "Hello World"));
// true since s1 and s2 is the reference of the same object
System.out.println("s1 and s2: " + (s1 == s2));
// false since s3 is refered to another new object
System.out.println("s1 and s3: " + (s1 == s3));
// compare using 'equals'
System.out.println("Compared by 'equals':");
System.out.println("s1 and \"Hello World\": " + s1.equals("Hello World"));
System.out.println("s1 and s2: " + s1.equals(s2));
System.out.println("s1 and s3: " + s1.equals(s3));
// compare using 'compareTo'
System.out.println("Compared by 'compareTo':");
System.out.println("s1 and \"Hello World\": " + (s1.compareTo("Hello World") == 0));
System.out.println("s1 and s2: " + (s1.compareTo(s2) == 0));
System.out.println("s1 and s3: " + (s1.compareTo(s3) == 0));
}
}
不可变字符串 — 问题和解决方案
在 Java 中,由于字符串是不可变的,因此在连接时首先为新字符串分配足够的空间,复制旧字符串中的内容并附加到新字符串。
- 解决方案
- 如果希望字符串是可变的,可以将其转换成字符数组。
代码:
public class Main {
public static void main(String[] args) {
String s = "Hello World";
char[] str = s.toCharArray();
str[5] = ',';
System.out.println(str);
}
}
- 如果你经常必须连接字符串,最好使用一些其他的数据结构,如 StringBuilder 。
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();
}
}