lc28-Implement strStr()

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Implement strStr().

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

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class Solution {
public:
    int strStr(string haystack, string needle) {
        if (needle == "")   return 0;
        if (haystack == "") return -1;
        int l = haystack.length();
        int ll = needle.length();
        int i = 0;
        while (i <= l-ll) {
            int j = i;
            while (j < l && j-i < ll && haystack[j] == needle[j-i]) {
                j++;
            }
            if (j-i == ll)  return i;
            i++;
        }
        return -1;
    }
};