这一部分的算法和链表联系的比较紧密，只要链表弄明白了，相信栈的算法也不会太难理解的。。。

栈算法的代码如下：

//
//  main.m
//  栈程序
//
//  Created by 小伴 on 16/7/24.
//  Copyright © 2016年 huangqimeng. All rights reserved.
//

#import <Foundation/Foundation.h>

#include <stdio.h>
#include <malloc/malloc.h>
#include <stdlib.h>

typedef struct Node {
    int data;
    struct Node *pNext;
}NODE, *PNODE;

typedef struct Stack {
    PNODE pTop;
    PNODE pBottom;
}STACK, *PSTACK;  //PSTACK等价于 struct STACK *

void initStack(PSTACK);
void pushStack(PSTACK, int);
void traverse(PSTACK);
bool popStack(PSTACK, int *);
void clearStack(PSTACK);

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        STACK S;  //STACK 等价于 struct Stack
        int val;

        initStack(&S);
        pushStack(&S, 1);
        pushStack(&S, 2);
        pushStack(&S, 3);
        pushStack(&S, 4);
        pushStack(&S, 5);
        traverse(&S);

        if (popStack(&S, &val)) {
            printf("出栈成功，出栈的元素为：%d\n", val);
        } else {
            printf("出栈失败\n");
        }
        traverse(&S);

        clearStack(&S);
        traverse(&S);
    }
    return 0;
}

/**
 * 初始化一个空栈
 */
void initStack(PSTACK pS) {
    pS->pTop = (PNODE)malloc(sizeof(NODE));
    if (pS->pTop == NULL) {
        printf("动态内存分配失败\n");
        exit(-1);
    } else {
        pS->pBottom = pS->pTop;
        pS->pTop->pNext = NULL;  //pS->pBottom->pNext = NULL;
    }
}

/**
 *  压栈
 */
void pushStack(PSTACK pS, int val) {
    PNODE pNew = (PNODE)malloc(sizeof(NODE));

    pNew->data = val;
    pNew->pNext = pS->pTop; //pS->pTop不能换作pS->pBottom
    pS->pTop = pNew;
}

void traverse(PSTACK pS) {
    PNODE p = pS->pTop;
    while (p != pS->pBottom) {
        printf("%d ", p->data);
        p = p->pNext;
    }
    printf("\n");
}

bool empty(PSTACK pS) {
    if (pS->pTop == pS->pBottom) {
        return true;
    } else {
        return false;
    }
}

/**
 *  把pS指向的栈出栈一次，并把出栈的元素存入pVal形参所指向的变量中，如果出栈失败返回false，否则返回true
 */
bool popStack(PSTACK pS, int *pVal) {
    if (empty(pS)) {//pS本身存放的就是S的地址
        return false;
    } else {
        PNODE temp = pS->pTop;
        *pVal = temp->data;
        pS->pTop = temp->pNext;
        free(temp);
        temp = NULL;

        return true;
    }
}

//栈清空
void clearStack(PSTACK pS) {
    if (!empty(pS)) {
        PNODE p = pS->pTop;
        PNODE q = NULL;
        while (p != pS->pBottom) {
            q = p->pNext;
            free(p);
            p = q;
        }
        pS->pTop = pS->pBottom;
    }
}