面试必会函数源代码 strcpy/memcpy/atoi/kmp/quicksort

http://blog.csdn.net/liuqiyao_01/article/details/26967813

 

二、stl模板函数

    1、strcpy

char * strcpy( char *strDest, const char *strSrc )
{
    if(strDest == strSrc) { return strDest; }
    assert( (strDest != NULL) && (strSrc != NULL) );
    char *address = strDest;
    while( (*strDest++ = * strSrc++) != '' );
    return address;
}

    2、strncpy

<span style="font-size:18px;">char *strncpy(char *strDes, const char *strSrc, unsigned int count)
{
    assert(strDes != NULL && strSrc != NULL);
    char *address = strDes;
    while (count-- && *strSrc != '')
        *strDes++ = *strSrc++;
    *strDes = '';
    return address;
} </span>

    3、strcmp

int strcmp(const char *s, const char *t)
{
    assert(s != NULL && t != NULL);
    while (*s && *t && *s == *t)
    {
        ++ s;
        ++ t;
    }
    return (*s - *t);
}

    4、strcat

char *strcat(char *strDes, const char *strSrc)
{
    assert((strDes != NULL) && (strSrc != NULL));
    char *address = strDes;
    while (*strDes != '')
        ++ strDes;
    while ((*strDes ++ = *strSrc ++) != '')
        NULL;
    return address;
}

    5、strlen

int strlen(const char *str)
{
    assert(str != NULL);
    int len = 0;
    while (*str ++ != '')
        ++ len;
    return len;
}

    6、strstr

char *strstr(const char *strSrc, const char *str)
{
    assert(strSrc != NULL && str != NULL);
    const char *s = strSrc;
    const char *t = str;
    for (; *strSrc != ''; ++ strSrc)
    {
        for (s = strSrc, t = str; *t != '' && *s == *t; ++s, ++t)
            NULL;
        if (*t == '')
            return (char *) strSrc;
    }
    return NULL;
}

    7、strncat

char *strncat(char *strDes, const char *strSrc, unsigned int count)
{
    assert((strDes != NULL) && (strSrc != NULL));
    char *address = strDes;
    while (*strDes != '')
        ++ strDes;
    while (count -- && *strSrc != '' )
        *strDes ++ = *strSrc ++;
    *strDes = '';
    return address;
}

    8、strncmp

int strncmp(const char *s, const char *t, unsigned int count)
{
    assert((s != NULL) && (t != NULL));
    while (*s && *t && *s == *t && count --)
    {
        ++ s;
        ++ t;
    }
    return (*s - *t);
}

    9、memcpy

void *memcpy(void *dest, const void *src, unsigned int count)
{
    assert((dest != NULL) && (src != NULL));
    void *address = dest;
    while (count --)
    {
        *(char *) dest = *(char *) src;
        dest = (char *) dest + 1;
        src = (char *) src + 1;
    }
    return address;
}

    10、memccpy

void *memccpy(void *dest, const void *src, int c, unsigned int count)
{
    assert((dest != NULL) && (src != NULL));
    while (count --)
    {
        *(char *) dest = *(char *) src;
        if (* (char *) src == (char) c)
            return ((char *)dest + 1);
        dest = (char *) dest + 1;
        src = (char *) src + 1;
    }
    return NULL;
}

    11、memcmp

int memcmp(const void *s, const void *t, unsigned int count)
{
    assert((s != NULL) && (t != NULL));
    while (*(char *) s && *(char *) t && *(char *) s == *(char *) t && count --)
    {
        s = (char *) s + 1;
        t = (char *) t + 1;
    }
    return (*(char *) s - *(char *) t);
}

    12、memmove

//@big：
//要处理src和dest有重叠的情况，不是从尾巴开始移动就没问题了。
//一种情况是dest小于src有重叠，这个时候要从头开始移动，
//另一种是dest大于src有重叠，这个时候要从尾开始移动。
void *memmove(void *dest, const void *src, unsigned int count)
{
    assert(dest != NULL && src != NULL);
    char* pdest = (char*) dest;
    char* psrc = (char*) src;

    //pdest在psrc后面，且两者距离小于count时，从尾部开始移动. 其他情况从头部开始移动
    if (pdest > psrc && pdest - psrc < count)
    {
        while (count--)
        {
            *(pdest + count) = *(psrc + count);
        }
    } else
    {
        while (count--)
        {
            *pdest++ = *psrc++;
        }
    }
    return dest;
}

    13、memset

void *memset(void *str, int c, unsigned int count)
{
    assert(str != NULL);
    void *s = str;
    while (count --)
    {
        *(char *) s = (char) c;
        s = (char *) s + 1;
    }
    return str;
}

三、atoi && itoa

    1、atoi

//代码自己所写，不对地方请及时指正，谢谢！
//inf用来标记作为判断是否越界
//atoiFlag作为是否是正确结果的返回值
const __int64 inf = (0x7fffffff);
int atoiFlag;
int atoi(const char* ch)
{
    ASSERT(ch != NULL);
    atoiFlag = false;
    while (*ch == ' ' || *ch == '	')
        ch ++;
    int isMinus = 1;
    if (*ch == '-')
    {
        isMinus = -1;
        ch ++;
    }
    else if (*ch == '+')
    {
        ch ++;
    }
    //判断非法
    if (!(*ch <= '9' && *ch >= '0'))
        return 0;
    __int64 ans = 0;
    while (*ch && *ch <= '9' && *ch >= '0')
    {
        ans *= 10;
        ans += *ch - '0';
        //判断越界
        if (ans > inf)
        {
            return inf;
        }
        ch ++;
    }
    ans *= isMinus;
    atoiFlag = true;
    return (int)ans;
}

//如何使用
int main()
{
    char a[100];
    while (scanf("%s", a) != EOF)
    {
        int ans = atoi(a);
        if (atoiFlag)
            printf("%d
", ans);
        else if (ans == inf)
            puts("The data of crossing the line");
        else
            puts("Not is a integer");
    }
    return 0;
}

    2、itoa

#include <stdlib.h>
#include <stdio.h>
char *myitoa(int num,char *str,int radix);
int main()
{
    int number = -123456;
    char string[25];
    myitoa(number, string, 16);
    printf("integer = %d string = %s
", number, string);
    return 0;
}
/* 实现itoa函数的源代码 */
char *myitoa(int num,char *str,int radix)
{
    /* 索引表 */
    char index[]="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    unsigned unum; /* 中间变量 */
    int i=0,j,k;
    /* 确定unum的值 */
    if(radix==10&&num<0) /* 十进制负数 */
    {
        unum=(unsigned)-num;
        str[i++]='-';
    }
    else unum=(unsigned)num; /* 其他情况 */
    /* 逆序 */
    do
    {
        str[i++]=index[unum%(unsigned)radix];
        unum/=radix;
    }while(unum);
    str[i]='';
    /* 转换 */
    if(str[0]=='-') k=1; /* 十进制负数 */
    else k=0;
    /* 将原来的“/2”改为“/2.0”，保证当num在16~255之间，radix等于16时，也能得到正确结果 */
    char temp;
    for(j=k;j<=(i-k-1)/2.0;j++)
    {
        temp=str[j];
        str[j]=str[i-j-1];
        str[i-j-1]=temp;
    }
    return str;
}

四、kmp  &&  quicksort

    1、kmp

 http://www.cnblogs.com/dolphin0520/archive/2011/08/24/2151846.html

void getNext(const char* p, int next[])
{
    next[0] = -1;
    int index = 0;
    int p_l = strlen(p);
    for (int i=1; i<p_l; i++)
    {
        index = next[i-1];
        while (index >= 0 && p[index+1] != p[i])
        {
            index = next[index];
        }
        if (p[index+1] == p[i])
        {
            next[i] = index + 1;
        }
        else
        {
            next[i] = -1;
        }
    }
}

int kmp(const char* t, const char* p)
{
    const int t_l = strlen(t);
    const int p_l = strlen(p);
    int Next = new int[p_l + 1];
    getNext(p, Next);
    int p_index = 0, t_index = 0;

    int cnt = 0;

    while (p_index < p_l && t_index < t_l)
    {
        if (t[t_index] == p[p_index])
        {
            t_index ++;
            p_index ++;
        }
        else if (p_index == 0)
        {
            t_index ++;
        }
        else
        {
            p_index = Next[p_index-1] + 1;
        }
        if (p_index == p_l)
        {
            cnt ++;
            p_index = Next[p_index-1] + 1;
        }
    }
    delete[] Next;
    return cnt;
}

    2、quicksort

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int partion(int a[], int p, int r)
{
    srand((unsigned)time(NULL));
    int e = rand()%(r-p+1)+p;
    int tmp = a[e];
    a[e] = a[r];
    a[r] = tmp;
    int x = a[r];
    int i, j;
    i = p;
    for (j=p; j<r; j++)
    {
        if (a[j] <= x)
        {
            tmp = a[i];
            a[i] = a[j];
            a[j] = tmp;
            i ++;
        }
    }
    tmp = a[r];
    a[r] = a[i];
    a[i] = tmp;
    return i;
}

void QuickSort(int a[], int p, int r)
{
    if (p < r)
    {
        int q = partion(a, p, r);
        QuickSort(a, p, q-1);
        QuickSort(a, q+1, r);
    }
}

int main()
{
    int a[] = {1,2,3,-1,5,-9,10,33,0};
    QuickSort(a, 0, 8);
    for (int i=0; i<9; i++)
    {
        printf("%d	", a[i]);
    }
    return 0;
}