LeetCode: Implement strStr()

思路：KMP经典算法，具体解释可以参照July的关于KMP的博文。

code:

class Solution {
public:
    void GetNext(char * needle, vector<int> &next){
        int n = strlen(needle);
        next[0] = -1;
        int k = -1;
        int j = 0;
        while(j < n-1){
            if(k == -1 || needle[j] == needle[k]){
                k++;
                j++;
                if(needle[j] != needle[k])
                    next[j] = k;
                else
                    next[j] = next[k];
            }
            else
                k = next[k];
        }
    }
    char *strStr(char *haystack, char *needle) {
        int n = strlen(haystack), m = strlen(needle);
        if(n==0){
            if(m == 0)
                return haystack;
            return NULL;
        }
        if(m == 0) return haystack;
        vector<int> next(m,0);
        GetNext(needle,next);
        int i = 0, j =0;
        while(i < n && j < m){
            char *p = haystack + i, *q = needle + j;
            if(j == -1 || *p == *q){
                i++;
                j++;
            }
            else
                j = next[j];
        }
        if(j >= m) 
            return haystack+i-j;
        else
            return NULL;
    }
};