[LeetCode] Implement strStr()

代码：

暴力算法

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

KMP算法

class Solution {
public:
    int strStr(char *haystack, char *needle) {
        return kmp(haystack, needle);
    }
private:
    int kmp(const char *text, const char *pattern) {
        int i = 0;
        int j = 0;
        const int n = strlen(text);
        const int m = strlen(pattern);
        int *next = (int*)malloc(sizeof(int)* m);
        compute_prefix(pattern, next);

        while (i < n && j < m) {
            if (j == -1 || text[i] == pattern[j]) {
                i++;
                j++;
            } else {
                j = next[j];
            }
        }

        free(next);
        if (j == m)
            return i - j;
        else
            return -1;
    }

    void compute_prefix(const char *pattern, int next[]) {
        int i = 0;
        int j = -1;
        const int m = strlen(pattern);
        next[0] = -1;

        while (i < m - 1) {
            if (j == -1 || pattern[i] == pattern[j]) {
                i++;
                j++;
                if (pattern[i] != pattern[j])
                    next[i] = j;
                else
                    next[i] = next[j];
            }
            else {
                j = next[j];
            }
        }
    }
};

杂记：

参考：http://blog.csdn.net/v_july_v/article/details/7041827