实验内容：

编程实现链表下教材第二章定义的线性表的基本操作，最好用菜单形式对应各个操作，使其编程一个完整的小软件。

实验步骤：

1.按照实验要求编写代码，构造链表，初始化链表
2.再插入元素1 2 3
3.分别求链表中元素的个数、求直接前驱、求直接后继
4.删除一个元素
5.查看链表中元素的个数并输出所有链表元素
6.初始化链表并重新输入链表元素
7.输出所输入的链表元素
8.最后销毁链表，退出。
9.输入验收用例，验证其输出结果。

#include <iostream>
#include <cstdlib>
#define False -1
using namespace std;
struct LNode {
    int data;
    LNode* next;
};

void InitList(LNode*& L) {
    L = NULL;
}

void DestroyList(LNode*& L) {
    LNode* p;
    while (L) {
        p = L;
        L = L->next;
        delete p;
    }
}

int ListLength(LNode* L) {
    int len = 0;
    LNode* p = L;
    while (p) {
        len++;
        p = p->next;
    }
    return len;
}

int GetElemValue(LNode* L, int i) {
    LNode* p = L;
    int j = 1;
    while (p && j < i) {
        p = p->next;
        j++;
    }
    if (!p || j > i) {
        cout << "错误: 非法位置！" << endl;
        return False;
    }
    return p->data;
}

int GetPosition(LNode* L, int e) {
    LNode* p = L;
    int pos = 1;
    while (p && p->data != e) {
        p = p->next;
        pos++;
    }
    if (!p) {
        cout << "错误: 非法位置！" << endl;
        return False;
    }
    return pos;
}

LNode* GetPredecessor(LNode* L, int e) {
    LNode* p = L;
    LNode* pred = NULL;
    while (p->next && p->next->data != e) {
        p = p->next;
    }
    if(p->next->data == e){
		pred = p;
	}
    return pred;
}

LNode* GetSuccessor(LNode* L, int e) {
    LNode* p = L;
    while (p && p->data != e) {
        p = p->next;
    }
    if (p && p->next) {
        return p->next;
    }
    return NULL;
}

bool ListInsert(LNode*& L, int i, int e) {
    if (i < 1) {
        cout << "错误: 非法位置！" << endl;
        return false;
    }
    LNode* p = new LNode;
    p->data = e;
    if (i == 1) {
        p->next = L;
        L = p;
    } else {
        LNode* q = L;
        int j = 1;
        while (q && j < i - 1) {
            q = q->next;
            j++;
        }
        if (!q || j > i - 1) {
            cout << "错误: 非法位置！" << endl;
            return false;
        }
        p->next = q->next;
        q->next = p;
    }
    return 0;
}

bool ListDelete(LNode*& L, int i) {
    if (i < 1) {
        cout << "错误: 非法位置！" << endl;
        return false;
    }
    if (i == 1) {
        LNode* p = L;
        L = L->next;
        delete p;
    } else {
        LNode* p = L;
        int j = 1;
        while (p && j < i - 1) {
            p = p->next;
            j++;
        }
        if (!p || !p->next || j > i - 1) {
            cout << "错误: 非法位置！" << endl;
            return false;
        }
        LNode* q = p->next;
        p->next = q->next;
        delete q;
    }
    return 0;
}

void showList(LNode* L) {
    LNode* p = L;
    while (p) {
        cout << p->data << " ";
        p = p->next;
    }
    cout << endl;
}

int main() {
    LNode* L1;
    InitList(L1);
	int a=0;
    int commander;
    while (true) {
        cout << "1. 初始化链表" << endl;
        cout << "2. 销毁链表" << endl;
        cout << "3. 求链表长度" << endl;
        cout << "4. 求指定位序的元素值" << endl;
        cout << "5. 求链表输入元素的位序" << endl;
        cout << "6. 求输入元素直接前驱" << endl;
        cout << "7. 求输入元素直接后继" << endl;
        cout << "8. 在第i个位置插入元素" << endl;
        cout << "9. 删除第i个元素" << endl;
        cout << "10. 输出所输入的链表元素" << endl;
        cout << "11. 初始化并输入链表元素" << endl;
        cout << "负数退出" << endl;
        cout << "请输入你的选择: ";
        cin >> commander;
		if(commander==1||commander==11){
			a++;
		}
		if(commander<1){
			break;
		}
		else{
			if(a==0){
				cout << "请先进行初始化操作！" << endl ;
				continue;
			}
		}
        switch (commander) {
            case 1:
                InitList(L1);
                break;
            case 2:
                DestroyList(L1);
                break;
            case 3:
                cout << "链表中数据元素个数为: " << ListLength(L1) << endl;
                break;
            case 4: {
                int i;
                cout << "请输入指定位序i: ";
                cin >> i;
                if(GetElemValue(L1, i)){
                cout << "指定位序的元素值为: " << GetElemValue(L1, i) << endl;
                }
                break;
            }
            case 5: {
                int e;
                cout << "请输入元素e: ";
                cin >> e;
                if(GetPosition(L1, e)){
                cout << "元素e的位序为: " << GetPosition(L1, e) << endl;
                }
                break;
            }
            case 6: {
                int e_pred;
                cout << "请输入元素e: ";
                cin >> e_pred;
                LNode* pred = GetPredecessor(L1, e_pred);
                if (pred) {
                    cout << "元素e的直接前驱为: " << pred->data << endl;
                } else {
                    cout << "未找到元素e的直接前驱！" << endl;
                }
                break;
            }
            case 7: {
                int e_succ;
                cout << "请输入元素e: ";
                cin >> e_succ;
                LNode* succ = GetSuccessor(L1, e_succ);
                if (succ) {
                    cout << "元素e的直接后继为: " << succ->data << endl;
                } else {
                    cout << "未找到元素e的直接后继！" << endl;
                }
                break;
            }
            case 8: {
                int i_insert, e_insert;
                cout << "请输入插入位置i: ";
                cin >> i_insert;
                cout << "请输入插入元素e: ";
                cin >> e_insert;
                ListInsert(L1, i_insert, e_insert);
                break;
            }
            case 9: {
                int i_delete;
                cout << "请输入删除位置i: ";
                cin >> i_delete;
                ListDelete(L1, i_delete);
                break;
            }
            case 10:
                cout << "链表元素为: ";
                showList(L1);
                break;
            case 11: {
                int n;
                cout << "请输入链表元素个数n: ";
                cin >> n;
                cout << "请输入链表元素: ";
                for (int j = 0; j < n; j++) {
                    int element;
                    cin >> element;
                    ListInsert(L1, j + 1, element);
                }
                break;
            }
            case 12:
                DestroyList(L1);
                exit(0);
            default:
                cout << "无效的输入，请重新输入！" << endl;
                break;
        }
        system("pause");
        system("cls");
    }

    return 0;
}