LeetCode-28. Implement strStr()

Implement strStr().

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

Example 1:

Input: haystack = "hello", needle = "ll"
Output: 2

Example 2:

Input: haystack = "aaaaa", needle = "bba"
Output: -1

Clarification:

What should we return when needle is an empty string? This is a great question to ask during an interview.

For the purpose of this problem, we will return 0 when needle is an empty string. This is consistent to C's strstr() and Java's indexOf().

这道题目显然是要考虑使用KMP算法的，那么现在就来写一个比较简单的KMP算法。

class Solution {
public:
     int strStr(string haystack, string needle) {
        int m = haystack.length(), n = needle.length();
        if (!n) {//如果子字符串为空
            return 0;
        }
        //kmp
        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;
    }
};