链表

链表是一种常见的重要的数据结构。 它是动态地进行存储分配的一种结构。 用数组存放数据时必须事先定义固定长度的数组， 如果不确定数组长度必须将数组定义的足够大， 于是很容易造成内存浪费。 而链表没有这种缺点， 它根据需要开辟内存单元。

链表是随机存储的， 在插入， 删除操作上有很高的效率。 但是如果要访问链表中的某个元素的话，那就得从链表的头逐个遍历，直到找到所需要的元素为止，所以链表的随机访问的效率就比数组要低

单链表的实现：

#include<stdio.h>
#include<iostream>

using namespace std;

struct NODE {
    int data;
    NODE *next;
};

NODE *head = NULL;

void Add() {
    NODE *p1, *p2;
    printf("please input the data\n >> ");
    int _data;
    cin >> _data;
    p1 = new NODE;
    if(head == NULL) {
        head = p1;
        p1 -> data = _data;
        p1 -> next = NULL;
    } else {
        p2 = head;
        while(p2 -> next != NULL) {
            p2 = p2 -> next;
        }
        p2 -> next = p1;
        p1 -> data = _data;
        p1 -> next = NULL;
    }
    cout << "successfully added" << endl;
}

void Delete() {
    NODE *p1, *p2;
    cout << "please input the data" << endl << " >> ";
    int _data;
    cin >> _data;
    bool is = false;
    if(head == NULL) {
        cout << "no data" << endl;
        is = true;
    }
    p2 = head;
    p1 = p2 -> next;
    if(p2 -> data == _data) {
        head = p2 -> next;
        cout << "successfully deleted" << endl;
        is = true;
    } else {
        while(p1 -> data != _data) {
            p1 = p1 -> next;
            p2 = p2 -> next;
        }
        p2 -> next = p1 -> next;
        cout << "successfully deleted" << endl;
        is = true;
    }
    if(is == false) {
        cout << "no this data!" << endl;
    }
}

void Change() {
    cout << "please input the old data" << endl << " >> ";
    int orgin, _data;
    cin >> orgin;
    cout << "please input the new data" << endl << " >> ";
    cin >> _data;
    NODE *p1;
    p1 = head;
    while(p1 -> data != orgin) {
        p1 = p1 -> next;
    }
    p1 -> data = _data;
    cout << "successfully changed!" << endl;
}

void Show() {
    NODE *p1;
    p1 = head;
    while(p1 != NULL) {
        cout << p1 -> data << " >> ";
        p1 = p1 -> next;
    }
    cout << endl;
}

int main() {
    while(1) {
        printf("1--add, 2--delete, 3--change, 4--show, 5--insert, 6--exit\n >> ");
        int i1;
        int flag = 0;
        cin >> i1;
        switch(i1) {
        case 1 :
            Add();
            break;
        case 2 :
            Delete();
            break;
        case 3 :
            Change();
            break;
        case 4 :
            Show();
            break;
        case 5 :
            Insert();
            break;
        case 6 :
            flag = 1;
            break;
        }
        if(flag)break;
    }
    return 0;
}