28. Find the Index of the First Occurrence in a String

EasyTwo PointersStringString Matching

Problem Description

The task is to find the first occurrence of the string needle within another string haystack. If needle is found within haystack, we should return the starting index of the first occurrence. If needle is not found, we return -1. It's important to note that an empty needle results in 0, as an empty string is considered to be a substring of any string, including an empty string itself.

Intuition

To solve this problem, the intuitive approach is to scan through the haystack string and for each position, check whether the substring starting at that position matches the needle. We can do this in a linear scan, considering the length of needle is m and the length of haystack is n.

We only need to scan until n - m + 1 in haystack, since if we start matching any later than that, needle would overflow the bounds of haystack. For each position i starting from 0 to n - m, we take a substring of haystack from i to i + m and compare it against needle. If it matches, we know we've found the first occurrence, and we return the index i. If we reach the end of the scan without finding needle, we return -1.

The time complexity of this approach is O((n-m) x m) since in the worst-case scenario, for each starting position, we might compare m characters until we find a mismatch. The space complexity is O(1) as we are not using any extra space proportional to the input size; we are only using a few variables to store the indices and lengths.

Learn more about Two Pointers patterns.

Solution Approach

The implementation of the solution is straightforward, following the idea described in the intuition. Here's a step-by-step explanation of the algorithm and its practical realization in the given Python code:

First, we obtain the length of both haystack and needle to manage our loop and comparisons. Let's denote the length of haystack as n and the length of needle as m.
We use a single loop to iterate over the haystack string. The end condition for our loop is n - m + 1, which ensures that we don't attempt to match needle beyond the point where it could possibly fit inside haystack.
Inside the loop, we take a substring of haystack starting from the current index i and spanning m characters (the entire length of needle). In Python, the substring operation is haystack[i : i + m].
We then compare this substring with needle. If they are equivalent, it means we have found the first occurrence of needle in haystack. In this case, we return the current index i.
If the loop completes without finding a match, it means needle is not a part of haystack. In this final case, we return -1 to indicate the absence of needle in haystack.

In terms of algorithms, this approach uses a simple linear scan with a direct string comparison, making it easy to understand and implement. No additional data structures or complex patterns are used, keeping the space complexity to O(1).

The key part of the code that performs the above logic is:

for i in range(n - m + 1):
    if haystack[i : i + m] == needle:
        return i
return -1

This code reflects directly the described steps, iterating through haystack, extracting substrings, and comparing them with needle. It's a simple yet effective solution that leverages Python's built-in ability to handle substrings and comparisons elegantly.

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Example Walkthrough

Let's walk through a small example to illustrate the solution approach.

Consider the following strings:

haystack = "hello"
needle = "ll"

Now, following the solution approach steps:

Obtain lengths of haystack and needle. Here, n = 5 (length of "hello") and m = 2 (length of "ll").
Begin a loop to iterate over the haystack from index 0 to 5 - 2 + 1 = 4.
Inside the loop, extract substrings of haystack of length m. We will have the following comparisons:
- i = 0 -> compare "he" with "ll" -> not a match
- i = 1 -> compare "el" with "ll" -> not a match
- i = 2 -> compare "ll" with "ll" -> this is a match!
Since we found the match "ll" in haystack starting at index 2, we can stop our search and return this index.
If no match was found by the end of the loop, we would return -1. However, in this case, we did find the needle in the haystack, so the loop ceases with a successful outcome, returning 2.

The loop would have continued to i = 3 and compared "lo" with "ll" if a match had not been found at i = 2.

Following the solution approach, this process efficiently finds the first occurrence of needle in the haystack, if it exists, and returns its starting index. If the needle is not present, -1 would be the result.

Solution Implementation

1class Solution:
2    def strStr(self, haystack: str, needle: str) -> int:
3        # Length of the haystack and needle strings
4        haystack_length, needle_length = len(haystack), len(needle)
5
6        # Check all possible starting positions of needle in haystack
7        for start in range(haystack_length - needle_length + 1):
8            # If the substring matching the needle's length equals the needle, return the start index
9            if haystack[start : start + needle_length] == needle:
10                return start
11      
12        # If the needle is not found in haystack, return -1
13        return -1
14
15# The method strStr is intended to find the first occurrence of the needle string
16# in the haystack string and return the index at which it begins.
17# If needle is not part of haystack, it returns -1.
18

1class Solution {
2    public int strStr(String haystack, String needle) {
3        // If needle is empty, the index to return is 0 (as per the problem statement).
4        if (needle.isEmpty()) {
5            return 0;
6        }
7
8        // Get the lengths of haystack and needle.
9        int haystackLength = haystack.length();
10        int needleLength = needle.length();
11
12        // Initialize pointers for haystack and needle.
13        int haystackPointer = 0;
14        int needlePointer = 0;
15
16        // Iterate through the haystack.
17        while (haystackPointer < haystackLength) {
18            // Check if the current characters in the haystack and needle are the same.
19            if (haystack.charAt(haystackPointer) == needle.charAt(needlePointer)) {
20                // Special case: if needle length is 1 and characters match, we found the needle.
21                if (needleLength == 1) {
22                    return haystackPointer;
23                }
24                // Move both pointers forward.
25                haystackPointer++;
26                needlePointer++;
27            } else {
28                // Current characters do not match. Reset haystackPointer back by the amount
29                // needlePointer had advanced, then move forward by one to search from next position.
30                haystackPointer -= needlePointer - 1;
31                // Reset needlePointer back to the start of the needle.
32                needlePointer = 0;
33            }
34
35            // Check if the needle has been found within the haystack.
36            if (needlePointer == needleLength) {
37                // The start of the substring in haystack that matches the needle
38                // is at the difference between current haystackPointer and the length of the needle.
39                return haystackPointer - needlePointer;
40            }
41        }
42      
43        // Needle was not found in the haystack. Return -1 as specified in the problem statement.
44        return -1;
45    }
46}
47

1class Solution {
2private:
3    // Constructs the 'next' array used in the KMP algorithm to optimize matching
4    vector<int> buildNextArray(string pattern) {
5        vector<int> next(pattern.length());
6        next[0] = -1; // Initialization with -1 for the first character
7        int index = 0; // Index in the pattern string
8        int prefixIndex = -1; // Index of the longest prefix that is also a suffix
9        int patternLength = pattern.length();
10        while (index < patternLength) {
11            // When there is a mismatch or it's the first iteration
12            while (prefixIndex >= 0 && pattern[index] != pattern[prefixIndex])
13                prefixIndex = next[prefixIndex];
14            index++, prefixIndex++;
15            // If we have not reached the end of the pattern
16            if (index < patternLength) {
17                // Record the length of the longest prefix which is also suffix
18                if (pattern[index] == pattern[prefixIndex])
19                    next[index] = next[prefixIndex];
20                else
21                    next[index] = prefixIndex;
22            }
23        }
24        return next;
25    }
26
27public:
28    // Function to find the first occurrence of `needle` in `haystack`
29    int strStr(string haystack, string needle) {
30        if (needle.empty()) // If the needle is empty, return 0 as per convention
31            return 0;
32
33        vector<int> nextArray = buildNextArray(needle);
34
35        int haystackLength = haystack.length(); // Length of the haystack string
36        int needleLength = needle.length(); // Length of the needle string
37        int len = haystackLength - needleLength + 1; // Compute the limit of searching
38        for (int i = 0; i < len; ++i) {
39            int needleIndex = 0; // Index for the needle string
40            int haystackIndex = i; // Starting index in the haystack string
41            // Search while the characters match and we are within both strings
42            while (needleIndex < needleLength && haystackIndex < haystackLength) {
43                if (haystack[haystackIndex] != needle[needleIndex]) {
44                    if (nextArray[needleIndex] >= 0) {
45                        needleIndex = nextArray[needleIndex];
46                        continue; // Use the next array to skip some comparisons
47                    } else
48                        break; // Mismatch without a valid sub-prefix match
49                }
50                ++haystackIndex, ++needleIndex;
51            }
52            // When the whole needle string has been traversed, return the starting index
53            if (needleIndex == needleLength)
54                return haystackIndex - needleIndex;
55        }
56
57        return -1; // If the needle has not been found, return -1
58    }
59};
60

1/**
2 * Finds the first occurrence of the `needle` in `haystack`, and returns its index.
3 * If `needle` is not found in `haystack`, returns -1.
4 * 
5 * @param haystack - The string to be searched within.
6 * @param needle - The string to find in the haystack.
7 * @returns The index at which the needle is found in the haystack, or -1 if not found.
8 */
9function strStr(haystack: string, needle: string): number {
10    // Length of the haystack and needle strings
11    const haystackLength: number = haystack.length;
12    const needleLength: number = needle.length;
13
14    // Early return if the needle is an empty string
15    if (needleLength === 0) return 0;
16
17    // Loop through each character in the haystack until there's no room left for the needle
18    for (let i = 0; i <= haystackLength - needleLength; i++) {
19        // Assume that the needle is found unless a mismatch is found
20        let isMatch: boolean = true;
21
22        // Check each character of the needle against the haystack
23        for (let j = 0; j < needleLength; j++) {
24            if (haystack[i + j] !== needle[j]) {
25                // If characters do not match, set isMatch to false and break out of the inner loop
26                isMatch = false;
27                break;
28            }
29        }
30
31        // If the needle was found in the haystack, return the starting index `i`
32        if (isMatch) {
33            return i;
34        }
35    }
36
37    // If the needle was not found in the haystack, return -1
38    return -1;
39}
40

Time and Space Complexity

The time complexity of the provided code is O((n - m + 1) * m), where n is the length of the haystack string and m is the length of the needle string. The for loop iterates up to (n - m + 1) times for the worst-case scenario, and the == operation inside the loop takes O(m) time to compare the substring to the needle.

The space complexity of the code is O(1) since only a few variables are used and there is no additional space allocated that is dependent on the input size.

Learn more about how to find time and space complexity quickly using problem constraints.

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