【LeetCode & 剑指offer 刷题笔记】目录（持续更新中...）
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().
//问题：实现strstr函数，子串查找（子串匹配）
//方法一：扫描主串，再扫描子串，O（mn）
#include <string>
class Solution
{
public:
int strStr(string haystack, string needle)
{
int m = haystack.size(), n = needle.size();
if(n == 0) return 0; //如果needle为空
/* int result = haystack.find(needle);
if(result != string::npos) return result;
else return -1;*/
for(int i = 0; i <= m-n; i++) //扫描主串，最后一个位置起始索引为m-n, i = 0~m-n
{
int j =0;
for(; j<n; j++) //扫描子串,j=0~n-1
{
if(haystack[i+j] != needle[j]) break; //如果有某个字符不匹配则退出循环
}
if(j == n) return i; //如果均匹配，返回子串在主串中的起始索引
}
return -1; //如果未找到，则返回-1
}
};
//方法二：KMP算法（子串匹配的更高效算法）