访谈将源代码的函数 strcpy/memcpy/atoi/kmp/quicksort

一、社论

继上一次发表了一片关于參加秋招的学弟学妹们怎样准备找工作的博客之后，反响非常大。顾在此整理一下，以便大家复习。好多源自july的这篇博客，也有非常多是我自己整理的。希望大家可以一遍一遍的写。直到可以秒杀为止。

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++) != '\0' );
    return address;
}  

2、strncpy

char *strncpy(char *strDes, const char *strSrc, unsigned int count)
{
    assert(strDes != NULL && strSrc != NULL);
    char *address = strDes;
    while (count-- && *strSrc != '\0')
        *strDes++ = *strSrc++;
    *strDes = '\0';
    return address;
} 

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 != '\0')
        ++ strDes;
    while ((*strDes ++ = *strSrc ++) != '\0')
        NULL;
    return address;
} 

5、strlen

int strlen(const char *str)
{
    assert(str != NULL);
    int len = 0;
    while (*str ++ != '\0')
        ++ 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 != '\0'; ++ strSrc)
    {
        for (s = strSrc, t = str; *t != '\0' && *s == *t; ++s, ++t)
            NULL;
        if (*t == '\0')
            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 != '\0')
        ++ strDes;
    while (count -- && *strSrc != '\0' )
        *strDes ++ = *strSrc ++;
    *strDes = '\0';
    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。不是从尾巴開始移动就没问题了。
//一种情况是dest小于src有重叠。这个时候要从头開始移动。
//还有一种是dest大于src有重叠，这个时候要从尾開始移动。
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。从尾部開始移动. 其它情况从头部開始移动
    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，不正确地方请及时指正。谢谢！
//inf用来标记作为推断是否越界
//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 == '\t')
		ch ++;
	int isMinus = 1;
	if (*ch == '-')
	{
		isMinus = -1;
		ch ++;
	}
	else if (*ch == '+')
	{
		ch ++;
	}
	//推断非法
	if (!(*ch <= '9' && *ch >= '0'))
		return 0;
	__int64 ans = 0;
	while (*ch && *ch <= '9' && *ch >= '0')
	{
		ans *= 10;
		ans += *ch - '0';
		//推断越界
		if (ans > inf)
		{
			return inf;
		}
		ch ++;
	}
	ans *= isMinus;
	atoiFlag = true;
	return (int)ans;
}

//怎样使用
int main()
{
	char a[100];
	while (scanf("%s", a) != EOF)
	{
		int ans = atoi(a);
		if (atoiFlag)
			printf("%d\n", 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\n", number, string);
	return 0;
}
/* 实现itoa函数的源码 */
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; /* 其它情况 */
	/* 逆序 */
	do
	{
		str[i++]=index[unum%(unsigned)radix];
		unum/=radix;
	}while(unum);
	str[i]='\0';
	/* 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、经典算法：kmp  &&  quicksort  &&  MergeSort

1、kmp

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\t", a[i]);
	}
	return 0;
}

3、MergeSort

#include<iostream.h>
void Merge(int r[], int r1[], int s, int m, int t)
{
	int i = s;
	int j = m+1;
	int k = s;
	while (i <= m && j <= t)
	{
		if (r[i] <= r[j])
			r1[k++] = r[i++];
		else
			r1[k++] = r[j++];
	}
	if (i <= m)
		while (i <= m)
			r1[k++] = r[i++];
	else
		while (j <= t)
			r1[k++] = r[j++];
	for(int n = s; n <= t; n++)
		r[n] = r1[n];
}

void MergeSort(int r[], int r1[], int s, int t)
{
	if (s < t)
	{
		int m = (s+t)/2;
		MergeSort(r, r1, s, m);
		MergeSort(r, r1, m+1, t);
		Merge(r, r1, s, m, t);
	}
}
int main()
{
	int r[8] = {10, 3, 5, 1, 9, 34, 54, 565}, r1[8];
	MergeSort(r, r1, 0, 7);
	for(int q = 0; q<8; q++)
		cout<<" "<<r[q];
	return 0;
}

2、quicksort