28. Find the Index of the First Occurrence in a String

Description

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.

Example 1:

Input: haystack = "sadbutsad", needle = "sad"
Output: 0
Explanation: "sad" occurs at index 0 and 6.
The first occurrence is at index 0, so we return 0.

Example 2:

Input: haystack = "leetcode", needle = "leeto"
Output: -1
Explanation: "leeto" did not occur in "leetcode", so we return -1.

Constraints:

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

Solution

find-the-index-of-the-first-occurrence-in-a-string.py

class Solution:
    def strStr(self, haystack: str, needle: str) -> int:
        if needle == "" or haystack == needle: return 0

        n = len(needle)
        s = deque()

        for index, x in enumerate(haystack):
            if len(s) < n:
                s.append(x)

            if len(s) == n:
                word = "".join(s)
                if word == needle:
                    return index - n + 1

                s.popleft()

        return -1

find-the-index-of-the-first-occurrence-in-a-string.java

class Solution {
   public int strStr(String haystack, String needle) {
      for (int i = 0; ; i++) {
        for (int j = 0; ; j++) {
          if (j == needle.length()) return i;
          if (i + j == haystack.length()) return -1;
          if (needle.charAt(j) != haystack.charAt(i + j)) break;
        }
      }
    }
}