【LeetCode 28_字符串_匹配】Implement strStr()

解法一：Brute-force

int strStr(string haystack, string needle)
{
    int m = haystack.size();
    int n = needle.size();
    if (!n) 
        return 0;
    for (int i = 0; i < m - n + 1; ++i) {
        int k = i, j = 0;
        while (j < n) {
            if (needle[j] == haystack[k]) {
                j++;
                k++;
            } else {
                break;
            }
        }
        if (j == n)
            return i;
    }
    return -1;
}

解法二：KMP

vector<int> GetNext(const string& T)
{
    int len = T.size();
    vector<int> next(len, 0);
    for (int i = 1, k = 0; i < len;) {
        if (T[i] == T[k])
            next[i++] = ++k;
        else if (k)
            k = next[k - 1];
        else
            next[i++] = 0;
    }
    return next;
}

int strStr(string haystack, string needle)
{
    int m = haystack.size();
    int n = needle.size();
    if (n == 0)
        return 0;
    
    vector<int> next = GetNext(needle);
    for (int i = 0, j = 0; i < m;) {
        if (haystack[i] == needle[j]) {
            i++;
            j++;
        }
        if (j == n)
            return i - n;
        if (i < m && haystack[i] != needle[j]) {
            if (j)
                j = next[j - 1];
            else
                i++;
        }
    }
    return -1;
}