POJ 2752

错误：

1. z-algorithm 实现错误

2. 忘记给z[]清零

3. KMP 更容易

void z_algorithm(char *s, int n) {
    int L = 0, R = 0;
    for (int i = 1; i < n; ++i) {
        if (i > R) {
            L = R = i;
            while (R < n && s[R - L] == s[R]) ++R;
            z[i] = R - L;
            --R;
        } else {
            int k = i - L;
            if (z[k] < R - i + 1)
                z[i] = z[k];
            else {
                L = i;
                while (R < n && s[R - L] == s[R]) ++R;
                z[i] = R - L;
                --R;
            }
        }
    }
}
vector<int> z_function(string s) {
    int n = (int) s.length();
    vector<int> z(n);
    for (int i = 1, l = 0, r = 0; i < n; ++i) {
        if (i <= r)
            z[i] = min (r - i + 1, z[i - l]);
        while (i + z[i] < n && s[z[i]] == s[i + z[i]])
            ++z[i];
        if (i + z[i] - 1 > r)
            l = i, r = i + z[i] - 1;
    }
    return z;
}