  [LeetCode][Easy] 28. Implement strStr()
2021-04-07
Engineering
2 MIN

## 題目

Implement strStr().

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 1:

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

Example 2:

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

Example 3:

``````Input: haystack = "", needle = ""
Output: 0
``````

## Solution

``````/**
* @param {string} haystack
* @param {string} needle
* @return {number}
*/
var strStr = function(haystack, needle) {
if (needle.length == 0) return 0

let l = 0
let r = needle.length - 1

while(r < haystack.length) {
if (haystack.substr(l, needle.length) == needle) {
return l
}
l++
r++
}
return -1
};
``````

## Complexity

• Time complexity : O(n)
• Space complexity : O(1)