[LintCode] 字符串查找

暴力解法（O(mn)）：

class Solution {
public:
    /**
     * Returns a index to the first occurrence of target in source,
     * or -1  if target is not part of source.
     * @param source string to be scanned.
     * @param target string containing the sequence of characters to match.
     */
    int strStr(const char *source, const char *target) {
        // write your code here
        if (!source || !target) return -1;
        int m = strlen(source), n = strlen(target);
        for (int i = 0; i < m - n + 1; i++) {
            int j = 0;
            for (; j < n; j++)
                if (source[i + j] != target[j])
                    break;
            if (j == n) return i;
        }
        return -1;
    }
};

KMP（O(m + n)）：

class Solution {
public:
    /**
     * Returns a index to the first occurrence of target in source,
     * or -1  if target is not part of source.
     * @param source string to be scanned.
     * @param target string containing the sequence of characters to match.
     */
    int strStr(const char *source, const char *target) {
        // write your code here
        if (!source || !target) return -1;
        int m = strlen(source), n = strlen(target);
        if (!n) return 0;
        vector<int> lps = kmpProcess(target);
        for (int i = 0, j = 0; i < m; ) {
            if (source[i] == target[j]) {
                i++;
                j++;
            }
            if (j == n) return i - j;
            if (i < m && source[i] != target[j]) {
                if (j) j = lps[j - 1];
                else i++;
            }
        }
        return -1;
    }
private:
    vector<int> kmpProcess(const char* target) {
        int n = strlen(target);
        vector<int> lps(n, 0);
        for (int i = 1, len = 0; i < n; ) {
            if (target[i] == target[len])
                lps[i++] = ++len;
            else if (len) len = lps[len - 1];
            else lps[i++] = 0;
        }
        return lps;
    }
};