LeetCode:Implement strStr()

標籤：leetcode   kmp   

Implement strStr().

Returns a pointer to the first occurrence of needle in haystack, or null if

needle is not part of haystack.

解題思路:

    strstr()函數隸屬標準庫string.h標頭檔，其內部的實現是O(n^2)的時間複雜度.在大師

克努特等人發明了KMP演算法過後，關於strstr()函數的實現的時間複雜度就是線性.對於KMP

演算法的理解，網上的文章一大堆，這裡就不做具體解釋.

解題代碼:

class Solution {public:    char *strStr(char *haystack, char *needle)     {        const int n = strlen(haystack), m = strlen(needle);                if (!m)            return haystack;        int next[m], i, j;        next[i = 0] = j = -1;        //求取next數組        while (i < m - 1)        {            if (j == -1 || needle[i] == needle[j])                next[++i] = ++j;            else                j = next[j];        }        //模式比對        for (i = j = 0; i < n;)        {            if (j == -1 || haystack[i] == needle[j])                ++i, ++j;            else                j = next[j];            if (j == m)                return haystack + i - m;        }        return NULL;    }};