Implement strStr() in Python

The strStr() function finds the first occurrence of a substring within a string and returns its index. This is similar to the strstr() function in C programming. If the substring is not found, it returns -1.

Algorithm

To implement strStr(), we use a two-pointer approach with the following steps ?

• Initialize i = 0 (for main string), j = 0 (for substring)
• If substring is empty, return 0
• While i = substring length:
  • If characters match, compare the entire substring
  • If complete match found, return starting index
  • Otherwise, move to next position in main string
• If no match found, return -1

Implementation

class Solution:
    def strStr(self, haystack, needle):
        """
        Find the first occurrence of needle in haystack
        :param haystack: Main string to search in
        :param needle: Substring to find
        :return: Index of first occurrence or -1 if not found
        """
        i = 0
        j = 0
        m = len(needle)
        n = len(haystack)
        
        if m == 0:
            return 0
            
        while i < n and n - i + 1 >= m:
            if haystack[i] == needle[j]:
                temp = i
                while j < m and i < n and needle[j] == haystack[i]:
                    i += 1
                    j += 1
                if j == m:
                    return temp
                i = temp + 1
                j = 0
            else:
                i += 1
        return -1

# Test the implementation
haystack = "helloworld"
needle = "lo"
solution = Solution()
result = solution.strStr(haystack, needle)
print(f"Index of '{needle}' in '{haystack}': {result}")

Index of 'lo' in 'helloworld': 3

Using Built-in Method

Python provides a simpler approach using the find() method ?

def strStr_builtin(haystack, needle):
    if not needle:
        return 0
    return haystack.find(needle)

# Test with different examples
test_cases = [
    ("helloworld", "lo"),
    ("programming", "gram"),
    ("python", "java"),
    ("", ""),
    ("abc", "")
]

for haystack, needle in test_cases:
    result = strStr_builtin(haystack, needle)
    print(f"'{needle}' in '{haystack}': {result}")

'lo' in 'helloworld': 3
'gram' in 'programming': 3
'java' in 'python': -1
'' in '': 0
'' in 'abc': 0

Comparison

Conclusion

The strStr() function can be implemented using a two-pointer approach to find substring occurrences. For practical applications, Python's built-in find() method provides the same functionality with cleaner syntax.