Q3.1 Describe how you could use a single array to implement three stacks

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Q:Describe how you could use a single array to implement three stacks

思路一：比较直观的想法是将一个数组分为三部分。将数组平分三部分，每个部分维护一个栈顶指针。对特定的栈进行操作只要用栈顶指针加上偏移量就可以。

思路二：思路一可能会浪费大量的空间。所以不均分数组为三部分。定义一种数据类型，可以记录当前元素值和上一个元素的位置。

#include <iostream>using namespace std;class stack3_1{private:int *buf;int size;int ptop[3];public:stack3_1(int size = 100) {this->size;buf = new int(size*3);ptop[0]=ptop[1]=ptop[2]=-1;}~stack3_1() {delete[] buf;}void push(int num, int val) {int ind = num*size + ptop[num] + 1;buf[ind] = val;ptop[num]++;}void pop(int num) {ptop[num]--;}int top(int num) {int ind = num*size + ptop[num];return buf[ind];}bool empty(int num) {return ptop[num] == -1;}};typedef struct node {int val;int preInd;node():val(0),preInd(-2){};}node;class stack3_2{private:node *buf;int size;int cur;int ptop[3];public:stack3_2(int size = 300) {this->size = size;buf = new node[size];ptop[0]=ptop[1]=ptop[2]=-1;cur = 0;}~stack3_2() {delete[] buf;}void push(int num, int val) {buf[cur].val = val;buf[cur].preInd = ptop[num];ptop[num] = cur;while (cur < this->size && buf[cur].preInd != -2) {cur++;}}void pop(int num) {if (cur > ptop[num]) {cur = ptop[num];}int ind = buf[ptop[num]].preInd;buf[ptop[num]].preInd = -2;ptop[num] = ind;}int top(int num) {return buf[ptop[num]].val;}bool empty(int num) {return ptop[num] == -1;}};int main(){    stack3_2 mystack;//stack3 mystack;    for(int i=0; i<10; ++i)        mystack.push(0, i);    for(int i=10; i<20; ++i)        mystack.push(1, i);    for(int i=100; i<110; ++i)        mystack.push(2, i);    for(int i=0; i<3; ++i)        cout<<mystack.top(i)<<" ";    cout<<endl;    for(int i=0; i<3; ++i){        mystack.pop(i);        cout<<mystack.top(i)<<" ";    }    mystack.push(0, 111);    mystack.push(1, 222);    mystack.push(2, 333);    for(int i=0; i<3; ++i)        cout<<mystack.top(i)<<" ";    return 0;}

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