我要好offer之 str/mem系列手写代码

1. str*系列手写代码

a. 一定要注意末尾‘\0‘的处理，切记切记

b. 一定要对输入做有效性判断，多用断言就是了

int Strlen(const char* str) {
    assert(str != NULL);
    const char* tmp = str;
    while (*tmp != ‘\0‘) {
        ++tmp;
    }
    return tmp - str;
}

char* Strcpy(char* dst, const char* src) {
    assert(dst != NULL && src != NULL);
    char* tmp = dst;
    while (*src != ‘\0‘) {
        *tmp++ = *src++;
    }
    *tmp = ‘\0‘;
    return dst;
}

char* Strncpy(char* dst, const char* src, int len) {
    assert(dst != NULL && src != NULL && len >= 0);
    char* tmp = dst;
    for (; len > 0 && *src != ‘\0‘; --len) {
        *tmp++ = *src++;
    }
    for (; len > 0; --len) {
        *tmp ++ = ‘\0‘;
    }
    return dst;
}

char* Strcat(char* dst, const char* src) {
    assert(dst != NULL && src != NULL);
    char* tmp = dst;
    while (*tmp != ‘\0‘) {
        ++tmp;
    }
    while (*src != ‘\0‘) {
        *tmp++ = *src++;
    }
    *tmp = ‘\0‘;
    return dst;
}

char* Strncat(char* dst, const char* src, int len) {
    assert(dst != NULL && src != NULL && n >= 0);
    char* tmp = dst;
    while (*tmp != ‘\0‘) {
        ++tmp;
    }
    for (; len > 0 && *src != ‘\0‘; --len) {
        *tmp++ = *src++;
    }
    *tmp = ‘\0‘;
    return dst;
}

int Strcmp(const char* str1, const char* str2) {
    assert(str1 != NULL && str2 != NULL);
    for (; *str1 == *str2; ++str1, ++str2) {
        if (*str1 == ‘\0‘) {
            return 0;
        }
    }
    if (*(unsigned char*)str1 < *(unsigned char*)str2) {
        return -1;
    } else {
        return 1;
    }
}

int Strncmp(const char* str1, const char* str2, int len) {
    assert(str1 != NULL && str2 != NULL && len >= 0);
    for (; len > 0; ++str1, ++str2) {
        if (*str1 != *str2) {
            return ((*(unsigned char*)str1) < (*(unsigned char*)str2) ? -1 : 1);
        } else if (*str1 == ‘\0‘) {
            return 0;
        }
    }
    return 0;
}

char* Strchr(const char* str, int c) {
    assert(str != NULL);
    const char* tmp = str;
    const char ch = (const char)c;
    for (; *tmp != ch; ++tmp) {
        if (*tmp == ‘\0‘) {
            return NULL;
        }
    }
    return (char*)tmp;
}

char* Strstr(const char* str1, const char* str2) {
    assert(str1 != NULL && str2 != NULL);
    if (*str2 == ‘\0‘) return str1;
    const char* tmp1 = str1;
    const char* tmp2 = str2;
    int len1 = Strlen(str1);
    int len2 = Strlen(str2);
    int i = 0;
    for (tmp1 = str1; i <= len1 - len2 && *tmp1 != ‘\0‘; ++tmp1, ++i) {
        if (*tmp1 != *tmp2)
            continue;
        const char* cur1 = tmp1;
        const char* cur2 = tmp2;

        while (*cur1 == *cur2) {
            ++cur1;
            ++cur2;
            if (*cur2 == ‘\0‘) {
                return (char*)tmp1;
            }
        }
    }
    return NULL;
}

2. mem系列手写代码

一定要对输入做有效性判断，多用断言就是了

void* Memchr(const void* src, int c, int len) {
    assert(src != NULL && len >= 0);
    const unsigned char ch = c;
    const unsigned char* tmp = (const unsigned char*)src;
    for (; len > 0; --len, ++tmp) {
        if (*tmp == ch) {
            return (void*)tmp;
        }
    }
    return NULL;
}

int Memcmp(const void* s1, const void* s2, int len) {
    assert(s1 != NULL && s2 != NULL);
    const unsigned char* tmp1 = (const unsigned char*)s1;
    const unsigned char* tmp2 = (const unsigned char*)s2;
    for (; len > 0; --len, tmp1++, tmp2++) {
        if (*tmp1 != *tmp2) {
            return ((*tmp1 < *tmp2) ? -1 : 1);
        }
    }
    return 0;
}

void* Memcpy(void* dst, const void* src, int len) {
    assert(dst != NULL && src != NULL && len >= 0);
    char* dstTmp = (char*)dst;
    const char* srcTmp = (const char*)src;
    for (; len > 0; --len, ++dstTmp, ++srcTmp) {
        *dstTmp = *srcTmp;
    }
    return dst;
}

void* Memmove(void* dst, const void* src, int len) {
    assert(dst != NULL && src != NULL && len >= 0);
    char* dstTmp = (char*)dst;
    const char* srcTmp = (const char*)src;
    if (dstTmp > srcTmp && dstTmp < srcTmp + len) {
        for (srcTmp += n, dstTmp += n; len > 0 ; --len, --srcTmp, --dstTmp) {
            *dstTmp = *srcTmp;
        }
    } else {
        for (; len > 0; --len, ++srcTmp, ++dstTmp) {
            *dstTmp = *srcTmp;
        }
    }
    return dst;
}

3. atoi函数

http://www.cnblogs.com/wwwjieo0/p/3687534.html

class Solution {
public:
    int atoi(const char *str) {
        assert(str != NULL);
        const char* curr = str;
        const int maxRange = 10;
        int tmp = 0;
        int num = 0;
        while(isspace(*curr)) {
            ++curr;
        }
        const char* start = nullptr;
        char sign;
        if (*curr == ‘-‘ || *curr == ‘+‘) {
            sign = *curr;
            ++curr;
            start = curr;
        }  else {
            start = curr;
        }

        while (isdigit(*curr)) {
            tmp = num;
            num = num * 10 + (*curr - ‘0‘);
            ++curr;
        }
        int len = 0;
        if (!isdigit(*curr)) {
            len = curr - start;
        }
        --curr;
        if (len > maxRange || num < num - *curr) {
            if (sign == ‘-‘) {
                return INT_MIN;
            } else {
                return INT_MAX;
            }
        }
        if (sign == ‘-‘)  num = -num;
        return num;
    }
};

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