28.ImplementStrStr.cpp

/*
 * @lc app=leetcode id=28 lang=cpp
 *
 * [28] Implement strStr()
 *
 * https://leetcode.com/problems/implement-strstr/description/
 *
 * algorithms
 * Easy (34.43%)
 * Likes:    1749
 * Dislikes: 2003
 * Total Accepted:    712K
 * Total Submissions: 2.1M
 * Testcase Example:  '"hello"\n"ll"'
 *
 * 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().
 * 
 * 
 * Constraints:
 * 
 * 
 * haystack and needle consist only of lowercase English characters.
 * 
 * 
 */

// @lc code=start
#include <string>

class Solution {
public:
    int strStr(std::string haystack, std::string needle) {
        if (needle.empty()) return 0;
        if (haystack.length() < needle.length()) return -1;
        for (int i = 0; i < haystack.length() - needle.length() + 1; i++) {
            int j;
            for (j = 0; j < needle.length(); j++) {
                if (haystack[i + j] != needle[j]) break;
            }
            if (j == needle.length()) return i;
        }
        return -1;
    }
};
// @lc code=end