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LeetCode 28 - Implement strStr()

  • Difficulty: Easy.
  • Related Topics: Two Pointers, String.
  • Link: leetcode

Problem​

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().

Solution​

/**
 * @param {string} haystack
 * @param {string} needle
 * @return {number}
 */
var strStr = function (haystack, needle) {
  const len = needle.length;
  if (len === 0) return 0;
  for (let i = 0; i < haystack.length; i++) {
    if (haystack.substring(i, i + len) === needle) {
      return i;
    }
  }
  return -1;
};

Complexity:

  • Time complexity : O(n*m).
  • Space complexity : O(1).