atoi()函数实现

实现atoi()函数，在出错时返回0；

int atoi(const char *str)
{
    if((str == NULL) || (strlen(str) == 0)) {
        return 0;
    }

    int value = 0;
    int sign = 0;
    int index = 0;
    int minMark = 0;
    const int MAX = 2147483647;
    const int MIN = -2147483648;
    while(str[index] != '\0') {
        char c = str[index];
        if((c == '+') || (c == '-')) {
            if(sign == 0) {
                sign = (c == '+' ? 1 : -1);
            }else {
                return 0;
            }
        }else if((c >= '0') && (c <= '9')) {
            if(value > MAX / 10) {
                return 0;
            }else if(value == MAX / 10) {
                if(c - '0' > MAX % 10 + 1) {
                    return 0;
                }else if((c - '0' == MAX % 10 + 1) && (sign == -1) && (minMark == 0)) {
                    minMark = 1;
                    value = value * 10 + MAX % 10;
                }else if(c - '0' <= MAX % 10) {
                    value = value * 10 + c - '0';
                }else {
                    return 0;
                }
            }else{
                value = value * 10 + c - '0';
            }
        }else {
            return 0;
        }
        index ++;
    }
    if(minMark == 1) {
        return MIN;
    }else if(sign == -1) {
        value *= -1;
    }
    return value;
}