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.

Update (2014-11-02):
The signature of the function had been updated to return the index instead of the pointer. If you still see your function signature returns a char * or String, please click the reload button to reset your code definition.

 

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

 

class Solution {
public:
    int strStr(char *S, char *T) {
        if(!S||!T)return -1;
        if(!*T)return 0;
        int Plen = strlen(T);
        int *next = new int[Plen+1];
        initKMP(T,next,Plen);
        int i=0,j = 0;//i for S,j for T
        for(i=0;j<Plen&&*(S+i);){
            if(S[i]==T[j]){
                i++;j++;
            }else if(next[j]>=0){
                j = next[j];
            }else{
                i++;
                j = 0;
                if(!*(S+i+Plen-1))
                    break;
            }
        }
        if(j>=Plen)return i-Plen;
        else return -1;
    }

    void initKMP(char str[],int next[],int len){
        int i=0,j=-1;
        next[0] = -1;
        while(i<len){
            while(j>=0&&str[i]!=str[j])
                j = next[j];
            i++;
            j++;
            if(str[j]==str[i])
                next[i] = next[j];
            else next[i] = j;
        }
    }
};