28. Implement strStr()

Implement strStr().

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

判断子串needle 在主串haystack中首次出现的位置

C++（6ms）：   KMP算法

class Solution {
public:
    vector<int> getNext(string needle , int tlen){
        int j, k;
        vector<int> Next(tlen+1, 0);
        j = 0; k = -1; Next[0] = -1;
        while(j < tlen)
            if(k == -1 || needle[j] == needle[k])
                Next[++j] = ++k;
            else
                k = Next[k];
        return Next ;
    }
    
    int strStr(string haystack, string needle) {
        int slen = haystack.size() ;
        int tlen = needle.size() ;
        if (!tlen)
            return 0 ;
        vector<int> Next = getNext(needle,tlen) ;
        int i = 0, j = 0;
        while(i < slen && j < tlen)
        {
            if(j == -1 || haystack[i] == needle[j])
            {
                i++; j++;
            }
            else
                j = Next[j];
        }
        if(j == tlen)
            return i - tlen ;
        else
            return -1;
    }
};