标签：style blog class code java c
Implement strStr().
Returns a pointer to the first occurrence of needle in haystack, or null if needle is not part of haystack.
此题我觉得并不是真要你写出kmp算法。 指针暴力法我觉得可能是考察点。而且要accept的话，必须要忽略后面一段不可能匹配的串。指针的操作要非常小心。
Of course, you can demonstrate to your interviewer that this problem can be solved using known efficient algorithms such as Rabin-Karp algorithm, KMP algorithm, and the Boyer-Moore algorithm. Since these algorithms are usually studied in advanced algorithms class, for an interview it is sufficient to solve it using the most direct method — The brute force method.
非指针代码很好小，很难有错。最长扫描n1-n2+1个就够了。
1 class Solution { 2 public: 3 char *strStr(char *haystack, char *needle) { 4 if (haystack == NULL || needle == NULL) return NULL; 5 int n1 = strlen(haystack); 6 int n2 = strlen(needle); 7 int j; 8 9 for (int i = 0; i < n1 - n2 + 1; ++i) { 10 j = 0; 11 while (j < n2 && needle[j] == haystack[i + j]) j++; 12 if (j == n2) return haystack + i; 13 } 14 return NULL; 15 } 16 };
c指针代码：
1 class Solution { 2 public: 3 char *strStr(char *haystack, char *needle) { 4 if (haystack == NULL || needle == NULL) return NULL; 5 if (*needle == ‘\0‘) return haystack; 6 char *ph = haystack, *pn = needle, *count = ph, *tmp; 7 while (*pn) { 8 if (!*count) return NULL; 9 pn++; 10 count++; 11 } 12 if (*needle) count--; 13 while (*count) { 14 pn = needle; 15 tmp = ph; 16 while (*pn && *tmp && *pn == *tmp) { 17 pn++; tmp++; 18 } 19 if (!*pn) return ph; 20 ph++; 21 count++; 22 } 23 24 return NULL; 25 } 26 };
如果是needle是空串，返回应该是haystack整个串。
最长扫描n1-n2+1个就够了。所以要让count循环m-1次。优化后的代码如下：
1 class Solution { 2 public: 3 char *strStr(char *haystack, char *needle) { 4 if (*needle == ‘\0‘) return haystack; 5 char *ph = haystack, *pn = needle, *count = ph, *tmp; 6 while (*++pn) { 7 if (!*count) return NULL; 8 count++; 9 } 10 while (*count) { 11 pn = needle; 12 tmp = ph; 13 while (*pn && *tmp && *pn == *tmp) { 14 pn++; tmp++; 15 } 16 if (!*pn) return ph; 17 ph++; 18 count++; 19 } 20 21 return NULL; 22 } 23 };
Leetcode | Implement strStr(),布布扣,bubuko.com
标签：style blog class code java c
原文地址：http://www.cnblogs.com/linyx/p/3728708.html
