字符串

比较两个字符串，用O(n)时间和恒量空间。
ANSWER:
What is “comparing two strings”? Just normal string comparison? The natural way use O(n) time and O(1) space.
int strcmp(char * p1, char * p2) {
  while (*p1 != ‘\0’ && *p2 != ‘\0’ && *p1 == *p2) {
    p1++, p2++;
  }
  if (*p1 == ‘\0’ && *p2 == ‘\0’) return 0;
  if (*p1 == ‘\0’) return -1;
  if (*p2 == ‘\0’) return 1;
  return (*p1 - *p2); // it can be negotiated whether the above 3 if’s are necessary, I don’t like to omit them.
}