# Leetcode28： Implement strStr() ### 题目描述

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().

### 代码

``````class Solution {
public:
int strStr(string haystack, string needle) {
if(haystack.length() < needle.length()){
return -1;
}

if(needle.empty()){
return 0;
}
else
{
int i = 0, j =0;
int count = 0;
//  cout << haystack.length() << "," << needle.length() <<endl;
while(i < (haystack.length()) && j < (needle.length()  )){

if(needle[j] == haystack[i]){
count++;
i++;
j++;
}else{
if(count != 0){
i = i - count + 1;
j = 0;
count = 0;
}else{
i++;
}
}
if(count == needle.length()){
return i - count;
}
//  cout << i << "," << j << "," << count <<endl;
}
return -1;
}

}
};
``````