实现atoi和itoa

/********************************
 * 实现atoi和itoa
 ********************************/
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
//将字符串转化为整数
int chartoi(char a)
{
    if(a >= '0' || a <= '9')
        return a - '0';
    else
        return 0;
}

int length(const char *str)
{
    int i = 0;
    while(str[i])
    {
        i++;
    }
    return i;
}

int myatoi(const char *str)
{
    int sum = 0;
    int len = length(str);
    if(str[0] == '-')
    {
        int i = 1;
        for(i = 1; i < len; i++)
        {
            sum = sum * 10 + chartoi(str[i]);
        }
        return 0 - sum;
    }
    else
    {
        int i = 0;
        for(i = 0; i < len; i++)
        {
            sum = sum * 10 + chartoi(str[i]);
        }
        return sum;
    }

}

//将整数转化为字符串
void reverse(char *str)
{
    int len = length(str);
    int i = 0;
    for(i = 0; i < len/2; i++)
    {
        char temp = str[i];
        str[i] = str[len - i -1];
        str[len - i -1] = temp;
    }
}

const char* neg_num(char *str)
{
    int len = length(str);
    int i = 0;
    for(i = len; i > 0; i--)
    {
        str[i] = str[i - 1];
    }
    str[0] = '-';
    return str;
}

const char* myitoa(int num)
{
    static char str[100];
    memset(str, 0, strlen(str));
    int neg = 0; //正负判断符
    if(num < 0)
    {
        neg = 0;
        num = 0 - num;
    }
    else
    {
        neg = 1;
    }

    int i = 0;
    while(num)
    {
        str[i] = num % 10 + 0x30;
        num = num / 10;
        i++;
    }
    reverse(str);
    if(neg == 0)
        return neg_num(str);
    else
        return str;
}

int main(void)
{
    printf("Hello World!\n");\

    printf("%s\n", myitoa(-1234556));
    return 0;
}