Basic Calculator II

Implement a basic calculator to evaluate a simple expression string.

The expression string contains only non-negative integers, +, -, *, / operators and empty spaces . The integer division should truncate toward zero.

You may assume that the given expression is always valid.

Some examples:
“3+2*2” = 7
” 3/2 ” = 1
” 3+5 / 2 ” = 5

說明：先將中綴表達式轉化爲後綴表達式，再由後綴表達式計算表達式的值。

代碼：

struct pri
{
    char op;
    int pri;
};

//設置符號的優先級，便於後續比較符號優先級
struct pri PriArray[5] = {{'=',0}, {'+',1}, {'-',1}, {'*',2}, {'/',2}};

//查找符號的優先級
int findpri(char c)
{
    int i = 0;
    for(i = 0; i < 5; i++)
    {
        if(c == PriArray[i].op)
            return PriArray[i].pri;
    }
    return 0;
}
//比較兩個符號的優先級(符號棧棧頂符號和遍歷中綴表達式的符號)
int pricom(char top, char tmp)
{
        if(findpri(top) < findpri(tmp))//棧頂符號優先級低，入棧
        return 1;
    else    //棧頂符號優先級高，出棧
        return -1;
}

//中綴表達式轉換爲後綴表達式
void transfer(char *s, char* postfix)
{
    char *opStack = (char *)malloc(sizeof(char) * strlen(s));  //存放中綴表達式符號的棧
    int top = -1;
    top++;
    char c;
    int i = 0, j = 0;

    opStack[top] = '=';

    while(s[i] != '\0')
    {
        c = s[i];
        if(c >= '0' && c <= '9')
        {
            while(s[i] >= '0' && s[i] <= '9')
            {
                postfix[j++] = s[i++];
            }
            postfix[j++] = '#';
        }
        else if(c == ' ')
        {
            i++;
        }
        else
        {
            switch(pricom(opStack[top], c)){
                case 1:
                    top++;
                    opStack[top] = c;
                    i++;
                    break;
                case -1:
                    while(pricom(opStack[top], c) == -1)
                    {
                        postfix[j++] = opStack[top];
                        top--;
                    }
                    top++;
                    opStack[top] = c;
                    i++;
                    break;
            }

        }
    }
    while(top > 0)    //將符號棧剩餘符號全部出棧
    {
        postfix[j++] = opStack[top];
        top--;
    }
    postfix[j] = '\0';
    free(opStack);
}

//計算表達式的值
int calculate(char* s) {
    int *num = (int *)malloc(sizeof(int) * strlen(s)); //存儲後綴表達式的數字
    int a = 0, b = 0;
    char *postfix = (char *)malloc(sizeof(char) * 2 * strlen(s)); //存儲後綴表達式
    transfer(s, postfix);
    //printf("%s\n",postfix);
    int top = -1, i = 0;
    int tmp;
    //由後綴表達式計算表達式的值
    while(postfix[i] != '\0')  
    {

        switch(postfix[i]){
            case '+':
                a = num[top--];
                b = num[top--];
                num[++top] = a + b;
                i++;
                break;
            case '-':
                a = num[top--];
                b = num[top--];
                num[++top] = b - a;
                i++;
                break;
            case '*':
                a = num[top--];
                b = num[top--];
                num[++top] = a * b;
                i++;
                break;
            case '/':
                a = num[top--];
                b = num[top--];
                num[++top] = b / a;
                i++;
                break;
            default:
                tmp = 0;
                while(postfix[i] >= '0' && postfix[i] <= '9')
                {
                    tmp = 10 * tmp + postfix[i] - '0';
                    i++;
                }
                num[++top] = tmp;
                if(postfix[i] == '#')
                    i++;
                break;
        }
    }
    i = num[top];
    free(num);
    return i;
}