题目

Implement strStr().

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

解题之法

class Solution {
public: 
    int strStr(string haystack, string needle) {
        int m = haystack.length(), n = needle.length();
        if (!n) return 0;
        for (int i = 0; i < m - n + 1; i++) {
            int j = 0;
            for (; j < n; j++)
                if (haystack[i + j] != needle[j])
                    break;
            if (j == n) return i;
        }
        return -1;
    }
};

分析

The problem does not aim for an advanced algorithm like KMP but only a clean brute-force algorithm. So we can traverse all the possible starting points of haystack (from 0 to haystack.length() - needle.length()) and see if the following characters in haystack match those of needle.