逆序字符串

记得最开始学编程的时候逆序字符串都是先malloc一个空间，然后将原字符串反向拷贝过去，但在面试或者笔试的时候不会太这么随意，都会给一些限制。
要求不得另外申请内存，即原地逆序：

int reverse_str(char *str){char *str_tail_p = str;char *str_head_p = str;char tmp = 0;if (str == NULL) {return -1;}while (*str_tail_p) {str_tail_p++;}/*指向最后一个元素*/str_tail_p -= 1;while(str_tail_p > str_head_p) {/*使用了临时变量*/tmp = *str_head_p;*str_head_p++ = *str_tail_p;*str_tail_p-- = tmp;}return 0;}

要求既不能另外申请内存也不能使用临时变量，那就只能依赖于异或了：

int reverse_str(char *str){char *str_tail_p = str;char *str_head_p = str;if (str == NULL) {return -1;}while (*str_tail_p) {str_tail_p++;}/*指向最后一个元素*/str_tail_p -= 1;while(str_tail_p > str_head_p) {/*使用异或运算*/*str_head_p = *str_head_p ^ *str_tail_p;*str_tail_p = *str_head_p ^ *str_tail_p;*str_head_p = *str_head_p++ ^ *str_tail_p--;}return 0;}

来个网上经常看到的题，以单词为单位对句子就行逆序，比如：
input：Say hello to the fucking world
output：world fucking the to hello Say
解题思路：
step1 先将每个单词进行逆序，yaS olleh ot eht gnikcuf dlrow
step2 将整个字符串整体逆序，world fucking the to hello Say

/*用始末指针标识单词范围*/void reverse_word(char *str_head, char *str_tail){char *str_head_p = str_head;char *str_tail_p = str_tail;while(str_tail_p > str_head_p) {*str_head_p = *str_head_p ^ *str_tail_p;*str_tail_p = *str_head_p ^ *str_tail_p;*str_head_p = *str_head_p++ ^ *str_tail_p--;}}int reverse_sentence(char *sentence){char *sentence_p = sentence;char *word_head_p = NULL;char *word_tail_p = NULL;if (sentence == NULL) {return -1;}/*找到第一个非空格字符*/while (*sentence_p) {if (*sentence_p == ' ') {sentence_p++;continue;}word_head_p = sentence_p;break;}/*全是空格*/if (!word_head_p) {return 0;}while (*sentence_p) {if (*sentence_p == ' ') {word_tail_p = sentence_p - 1;/*出现空格且空格前有单词*/reverse_word(word_head_p, word_tail_p);word_head_p = sentence_p + 1;}sentence_p++;}/*找到最后一个字符*/sentence_p -= 1;/*当原句为Say hello to the fucking world'\0'，即句子不是以空格结尾，**此时时最后一个单词还没有逆序*/if (*sentence_p != ' ') {word_tail_p = sentence_p;reverse_word(word_head_p, word_tail_p);}/*整个句子逆序*/reverse_word(sentence, sentence_p);return 0;}