CTDL&TT: Chuyển + Tính hậu tố

#include <stdio.h>
#include <iostream>
#include <string>
using namespace std;

#define MAX 200
struct Stack{
    float DATA[MAX];
    int TOP;
};

void init(Stack *S){
    S->TOP = -1;
}

int isEmpty(Stack *S){
    if (S->TOP == -1)
        return 1;
    return 0;
}

void Push(struct Stack* S, float item){
    if (S->TOP == (MAX - 1))
    {
        printf("\nStack is full");
    }
    else
    {
        ++S->TOP;
        S->DATA[S->TOP] = item;
    }
}

int top(Stack *S){
    return (S->DATA[S->TOP]);
}

float Pop(struct Stack* S){
    float ret = -1;
    if (S->TOP == -1)
        printf("\nStack is empty");
    else{
        ret = S->DATA[S->TOP];
        --S->TOP;
    }
    return ret;
}

int Precedence(char x){
    if (x == '(')
        return 0;
    if (x == '+' || x == '-')
        return 1 ;
    if (x == '*' || x == '/' || x == '%')
        return 2;
    return 3;
}


void InfixtoPostfix(char infix[], char postfix[]){
    Stack S;
    char x, token;
    int i=0, j=0;    //i-index of infix,j-index of postfix
    init(&S);
    for (i = 0; infix[i] != '\0'; i++)
    {
        token = infix[i];
        if (isalnum(token))
            postfix[j++] = token;
        else
            if (token == '(')
                Push(&S, '(');
            else
                if (token == ')')
                    while ((x = Pop(&S)) != '(')
                        postfix[j++] = x;
                else
                {
                    while (Precedence(token) <= Precedence(top(&S)) && !isEmpty(&S))
                    {
                        x = Pop(&S);
                        postfix[j++] = x;
                    }
                    Push(&S, token);
                }
    }

    while (!isEmpty(&S))
    {
        x = Pop(&S);
        postfix[j++] = x;
    }

    postfix[j] = '\0';
}
float Evaluate(char *Postfix)
{
    struct Stack S;
    char *p;
    float op1, op2, result;
    S.TOP = -1;
    p = &Postfix[0];
    while (*p != '\0')
    {
        while (*p == ' ' || *p == '\t')
        {
            p++;
        }
        if (isdigit(*p))
        {
            int num = 0;
            while (isdigit(*p))
            {
                num = num * 10 + *p - 48;
                *p++;
            }
            Push(&S, num);
        }
        else
        {
            op1 = Pop(&S);
            op2 = Pop(&S);
            switch (*p)
            {
            case '+':
                result = op2 + op1;
                break;
            case '-':
                result = op2 - op1;
                break;
            case '/':
                result = op2 / op1;
                break;
            case '*':
                result = op2 * op1;
                break;
            default:
                printf("\nInvalid Operator");
                return 0;
            }
            Push(&S, result);
        }
        p++;
    }
    result = Pop(&S);
    return result;
}

int main()
{
    char A[MAX], B[MAX];
    printf("Infix : ");
    gets(A);
    InfixtoPostfix(A, B);

    printf("Postfix: %s\n", B);
    printf("Equals is %f\n", Evaluate(&B[0]));
    gets(A);
    return 0;
}