LeetCode-28. Implement strStr()

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()​​.

题解：

很多测试用例没说清楚

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