---
title: 'LeetCode 28. Implement strStr()'
disqus: hackmd
---

# LeetCode 28. Implement strStr()

## Description

Implement strStr().

Given two strings needle and haystack, return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

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().

## Example

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

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

## Constraints

1 <= haystack.length, needle.length <= 104
haystack and needle consist of only lowercase English characters.

## Answer

此題可用雙層for求解，外層將haystack一個一個做平移檢查，內曾找符合needle字元的項目，若有跑到needle最後一個字元就值接return i。而第11行的return是當needle為空時就return 0，若不為空且跑到這行就表示完全不符合即return -1。

//2022_03_29

int strStr(char * haystack, char * needle){
    int i = 0, j = 0, lh = strlen(haystack), ln = strlen(needle);
    for(i = 0; i <= lh - ln; i++){
        for(j = 0; j < ln; j++){
            if(haystack[i + j] != needle[j]){break;}
            if(j == ln - 1){return i;}
        }
    }
    return ln == 0 ? 0 : -1;
}

## Link
https://leetcode.com/problems/implement-strstr/
###### tags: `Leetcode`