LeetCode-028：Implement strStr()

题目：

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().

题意：

判断第二个字符串是否是第一个字符串的子串，若是，返回第二个字符串第一个字符在第一个字符串出现的下标；若不是，返回-1；若第二个字符串为空字符串，返回0。

思路：

一道easy的题，就不去弄什么KMP、manacher算法了，简单点，暴力过

Code：

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