Problem Statement

Implement strStr().

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

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().

Example 1:

Input: haystack = "hello", needle = "ll"
Output: 2

Example 2:

Input: haystack = "aaaaa", needle = "bba"
Output: -1

Example 3:

Input: haystack = "", needle = ""
Output: 0

Constraints:

• 0 <= haystack.length, needle.length <= 5 * 104
• haystack and needle consist of only lower-case English characters.

Problem Link

Implement strStr() - LeetCode

Code

class Solution {
public:
    int strStr(string haystack, string needle) {
        
        int m = haystack.size();
        int n = needle.size();
        if(!n)
            return 0;
        for(int i=0;i<=m-n;i++)
        {
            if(haystack[i]==needle[0])
            {
                int j = i, k = 0;
                int flag = 1;
                while(j<m&&k<n)
                {
                    if(haystack[j++]!=needle[k++])
                    {
                        flag = 0;
                        break;
                    }
                }
                if(flag&&k==n)//all the chars of needle must be matched
                    return i;
            }
        }
        
        return -1;
    }
};