Implement strStr()

Implement strStr().

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

New version: 4ms

class Solution {
public:
    int strStr(string haystack, string needle) {
        int result = 0;
        int lenN = needle.size(), lenH = haystack.size();
        if(lenN == 0 && lenH == 0) return 0;
        if(lenN == 0) return 0;
        if(lenH < lenN) return -1;
        
        for(int i = 0; i <= lenH - lenN; i++){
            if(haystack[i] == needle[0]){
                int index = 0, move = i;
                while(move < lenH && index < lenN && haystack[move] == needle[index]){
                    move++;
                    index++;
                }
                if(index == lenN) return i;
            }
        }
        return -1;
    }
};

Analyse: Pay special attention to when the length of haystack is less  than that of needle, and the case that needle is empty.

class Solution {
public:
    int strStr(string haystack, string needle){
        int index = -1;
        if(haystack.length() < needle.length()) return index;
        else if(needle.length() == 0) return 0;
        
        for(int j = 0; j <= haystack.length() - needle.length(); j++){
            int index_needle = 0;
            if(haystack[j] == needle[index_needle]){
                index = j;
                for(int k = 1; k < needle.length(); k++){
                    if(haystack[j+k] != needle[index_needle+k]){
                        index = -1;
                        break;
                    }
                }
            }
            if(index >= 0) return index;
        }
        return index;
    }
};