Implement strStr() (Easy)

Description

Implement strStr().

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

Analysis

判断子串在父串中出现的第一个位置。直接kmp一下就行了

My Solution

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//C++
class Solution {
    void getnext(char *t,vector<int> &nxt){
        int tlen = strlen(t),i=0,j=-1;
        nxt.push_back(-1);
        while(i<tlen){
            if(j==-1||t[i]==t[j]){
                i++,j++;
                if(t[i]!=t[j])
                    nxt.push_back(j);
                else
                    nxt.push_back(nxt[j]);
            }
            else
                j = nxt[j];
        }
    }
public:
    int strStr(char *haystack, char *needle) {
        vector<int> nxt;
        nxt.clear();
        getnext(needle,nxt);
        int slen = strlen(haystack),tlen = strlen(needle);
        int i = 0,j=0;
        while(i<slen&&j<tlen){
            if(j==-1||needle[j]==haystack[i]){
                i++,j++;
            }
            else{
                j = nxt[j];
            }
        }
        if(j>=tlen)
            return i-tlen;
        return -1;
    }
};