Question

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()).

Difficulty:Easy

Category:String, Two Points

Analyze

Solution

class Solution {
 public:
  int strStr(string haystack, string needle) {
    if (needle.empty()) return 0;
    int len_m = haystack.size(), len_n = needle.size();
    if (len_m < len_n) return -1;
    for (int i = 0; i <= len_m - len_n; ++i) {
      int j = 0;
      for (j = 0; j < len_n; ++j) {
        if (haystack[i + j] != needle[j]) break;
      }
      if (j == len_n) return i;
    }
    return -1;
  }
};