[LeetCode]Implement strStr()

题目描述:(链接）

Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

解题思路：

class Solution {
public:
    int strStr(string haystack, string needle) {
        int sub_len = needle.size();
        int total_len = haystack.size();
        if (sub_len > total_len) {
            return -1;
        }
        
        if (haystack == needle) {
            return 0;
        }
        
        int result = -1;
        for (int i = 0; i <= total_len - sub_len; ++i) {
            if (haystack.substr(i, sub_len) == needle) {
                result = i;
                break;
            }
        }
        
        return result;
    }
};