字符串转换成整数

题目描述:
输入一个由数字组成的字符串，请把它转化成整数并输出.eg:输入字符串“123”，输出123
1分代码：

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int fun(int a,int b)
{
    if (b == 0) return 1;
    else
        return a*fun(a,b-1);
}
int strToInt(char *str)
{
    if (str == NULL)
        return -1;
    int i = 0,j = 0;
    int result = 0;
    int length = strlen(str);
    for (i = length-1;i>=0;i--)
    {
        int c = str[i] - '0';//刚开始还忽略了这一步
        result += c*fun(10,length-i-1);
        printf("result = %d\n",result);



    }
    return result;
}
int main()
{
    char str[1024];
    gets(str);
    int result = strToInt(str);
    printf("转化的整数是%d\n",result);
    return 0;
}

3‘程序：

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
/*int fun(int a,int b)
{
    if (b == 0) return 1;
    else
        return a*fun(a,b-1);
}*/
int strToInt(char *str)
{
    if (str == NULL)
        return -1;
    int i = 0,j = 0;
    int result = 0;
    int length = strlen(str);
    for (i = 0;str[i] != 0;i++)
    {
        int c = str[i] - '0';
        result = result * 10+c;
    }
    return result;
}
int main()
{
    char str[1024];
    gets(str);
    int result = strToInt(str);
    printf("转化的整数是%d\n",result);
    return 0;
}

10’程序：(处理溢出（最难的部分），空格，符号（‘+’，‘-’），空)

int strToInt(const char *str)
{
    static const int MAX_INT = (int)((unsigned)~0 >> 1);//~的优先级高于>>
    static const int MIN_INT = -(int)((unsigned)~0 >> 1);//
    unsigned int n = 0;
    if ( str == NULL)
    {
            return 0;
    }
    if (isspace(*str))//空格
    {
        ++str;
    }
    int sign = 1;
    if (*str == '-')
    {
        sign = -1;
        ++str;
    }
    char c;
    while (isdigit(*str))
    {
        c = *str - '0';
        if (sign > 0 && (n > MAX_INT/10 || (n == MAX_INT/10 && c > MAX_INT%10)))
        {
            n = MAX_INT;
            break;
        }
        if (sign < 0 && (n > (unsigned)MIN_INT/10 || (n == (unsigned)MIN_INT/10 && c > (unsigned)MIN_INT %10)))//这里 （unsigned）的使用
        {
            n = MIN_INT;
            break;
        }
        n = n*10+c;
        ++str;
    }
    return sign > 0?n:-n;
}