算法小记 · 字符串翻转

写一个函数，实现翻转字符串的功能，并考虑时间复杂度、空间复杂度，使其尽可能最小

1. 字符串翻转：请把"I am a student."，翻转成".tneduts a ma I"；

吾来打个样：

#include <stdio.h>
void reverseCharArray(char *str, char *begin, char *end) {
    while(begin < end) { //取地址，交换两个字符
        char temp = *begin;
        *begin = *end;
        *end = temp;
        begin++;
        end--;
    }
}

char *reverse_str(char *str) {
    if(NULL == str) { //字符串为空直接返回；
        return str;
    }
    char *begin;
    char *end;
    begin = end = str;
    
    while(*end != '\0') { //end指向字符串的末尾；
        end++;
    }
    --end;  //取出最后一个'.'字符；
    
    //取字符数组下标，进行交换；
    reverseCharArray(str, begin, end);
    
    return str; //返回结果；
}

int main(int argc, const char * argv[]) {
    char str[] = "I am a student.";
    printf("%s\n", reverse_str(str));
    return 0;
}

2. 字符串翻转：请把 "I am a student."，翻转成 "I ma a .tneduts"；

吾来打个样：

#include <stdio.h>
void reverseCharArray(char *str, char *begin, char *end) {
    while(begin < end) { //取地址，交换两个字符
        char temp = *begin;
        *begin = *end;
        *end = temp;
        begin++;
        end--;
    }
}

char *reverseChar(char *str) {
    char *begin;
    char *end;
    begin = end = str;
    while (*end != '\0') {
        if (*end == ' ') {
            reverseCharArray(str, begin, end - 1);
            begin = end + 1;
        }
        else if (*(end + 1) == '\0') {  //当*(end + 1)取出的是字符串数组最后的'\0'的时候
            reverseCharArray(str, begin, end);
        }
        end++;
    }
    return str;
}

int main(int argc, const char * argv[]) {
    char str[] = "I am a student.";
    printf("%s\n", reverseChar(str));
    return 0;
}

3. 字符串翻转：请把 "I am a student."，翻转为 "student. a am i"；

吾来打个样：

#include <stdio.h>
void reverseCharArray(char *str, char *begin, char *end) {
    while(begin < end) { //取地址，交换两个字符
        char temp = *begin;
        *begin = *end;
        *end = temp;
        begin++;
        end--;
    }
}

char *reverseChar(char *str) {
    char *begin;
    char *end;
    begin = end = str;
    while (*end != '\0') {
        if (*end == ' ') {
            reverseCharArray(str, begin, end - 1);
            begin = end + 1;
        }
        else if (*(end + 1) == '\0') {  //当*(end + 1)取出的是字符串数组最后的'\0'的时候
            reverseCharArray(str, begin, end);
        }
        end++;
    }
    return str;
}

int main(int argc, const char * argv[]) {
    char str[] = "I am a student.";
    char *begin;
    char *end;
    begin = end = str;

    while(*end != '\0') { //end指向字符串的末尾；
        end++;
    }
    --end;  //取出最后一个'.'字符；
    reverseCharArray(str, begin, end);
    printf("%s\n", reverseChar(str));
    return 0;
}