Implement strStr() -- leetcode

Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

Update (2014-11-02):
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算法一，Brute force

此算法在leetcode上的实际执行时间为16ms。

class Solution {public:    int strStr(char *haystack, char *needle) {        int i = 0, j = 0;        while (haystack[i] && needle[j]) {                if (haystack[i++] != needle[j++]) {                        i -= j-1;                        j = 0;                }        }        return !needle[j] ? i-j : -1;    }};

算法二，Rabin–Karp algorithm

特点是采取Rolling hash。

先计算好待查字符串needle的Hash值。然后将被搜索字符串haystack逻辑上分成若干组，每组长度与needle长度相同。每组分别计算Hash值。这样，比较字符串，变成了比较了比较Hash值。

所谓Rolling Hash，就是后一组的Hash计算，可以利用前一组的Hash成果。

因为后一组字符的组成与前一组的关系为，吐出尾部字符，吸纳一个新字符。这样计算Hash时，也回滚掉吐出的字符，吸纳进新的字符。

该算法在leetcode上实际执行时间为40ms。

class Solution {public:    int strStr(char *haystack, char *needle) {        const int q = 3355439;        const int r = 256;        int nLen = 0;        int nHash = 0;        int weight = 1;        while (*needle) {                nHash = ((nHash * r) % q + *needle) % q;                weight = nLen ? (weight * r) % q : 1;                ++needle;                ++nLen;        }        if (!nLen) return 0;        int k = nLen;        int hHash = 0;        while (*haystack && k) {                hHash = ((hHash * r) % q + *haystack) % q;                ++haystack;                --k;        }        if (k) return -1;        if (nHash == hHash &&                !strncmp(haystack-nLen, needle-nLen, nLen))                return 0;        int i = 1;        while (*haystack) {                hHash = ((hHash - (haystack[-nLen] * weight) % q) % q + q) % q;                hHash = ((hHash * r) % q + *haystack) % q;                if (nHash == hHash &&                        !strncmp(haystack-nLen+1, needle-nLen, nLen))                        return i;                ++haystack;                ++i;        }        return -1;    }};

参考资料

http://en.wikipedia.org/wiki/Rabin%E2%80%93Karp_algorithm

http://en.wikipedia.org/wiki/Rolling_hash

算法三：KMP

此算法在leetcode上实际执行时间为40ms。

class Solution {public:    int strStr(char *haystack, char *needle) {        vector<int> next;        preKMP(needle, next);        int i = 0, j= 0;        while (haystack[i] && needle[j]) {                while (j > -1 && needle[j] != haystack[i])                        j = next[j];                ++i;                ++j;        }        return !needle[j] ? i-j: -1;    }    void preKMP(char *needle, vector<int> &next) {        next.push_back(-1);        int i = 0;        int j = -1;        while (needle[i]) {                while (j > -1 && needle[i] != needle[j])                        j = next[j];                ++i;                ++j;                if (needle[i] == needle[j])                        next.push_back(next[j]);                else                        next.push_back(j);        }    }};

参考资料：

http://monge.univ-mlv.fr/~lecroq/string/node8.html

总结，Brute force执行效率远好于其它两种算法。怪不得leetcode将此题难度标注为easy，大约原意就是只考一考Brute fore。

毕竟高级字符串查找算法，只有在needle的长度越长时，才对Brute force有明显优势。