strstr.md

strStr

Question

• leetcode: Implement strStr() | LeetCode OJ
• lintcode: lintcode - (13) strstr

Problem Statement

For a given source string and a target string, you should output the first index(from 0) of target string in source string.

If target does not exist in source, just return -1.

Example

If source = "source" and target = "target", return -1.

If source = "abcdabcdefg" and target = "bcd", return 1.

Challenge

O(n2) is acceptable. Can you implement an O(n) algorithm? (hint: KMP)

Clarification

Do I need to implement KMP Algorithm in a real interview?

• Not necessary. When you meet this problem in a real interview, the interviewer may just want to test your basic implementation ability. But make sure your confirm with the interviewer first.

Problem Analysis

It's very straightforward to solve string match problem with nested for loops. Since we must iterate the target string, we can optimize the iteration of source string. It's unnecessary to iterate the source string if the length of remaining part does not exceed the length of target string. We can only iterate the valid part of source string. Apart from this naive algorithm, you can use a more effective algorithm such as KMP.

Python

class Solution:
    def strStr(self, source, target):
        if source is None or target is None:
            return -1

        for i in range(len(source) - len(target) + 1):
            for j in range(len(target)):
                if source[i + j] != target[j]:
                    break
            else:  # no break
                return i
        return -1

C

int strStr(char* haystack, char* needle) {
    if (haystack == NULL || needle == NULL) return -1;

    const int len_h = strlen(haystack);
    const int len_n = strlen(needle);
    for (int i = 0; i < len_h - len_n + 1; i++) {
        int j = 0;
        for (; j < len_n; j++) {
            if (haystack[i+j] != needle[j]) {
                break;
            }
        }
        if (j == len_n) return i;
    }

    return -1;
}

C++

class Solution {
public:
    int strStr(string haystack, string needle) {
        if (haystack.empty() && needle.empty()) return 0;
        if (haystack.empty()) return -1;
        if (needle.empty()) return 0;
        // in case of overflow for negative
        if (haystack.size() < needle.size()) return -1;

        for (int i = 0; i < haystack.size() - needle.size() + 1; i++) {
            string::size_type j = 0;
            for (; j < needle.size(); j++) {
                if (haystack[i + j] != needle[j]) break;
            }
            if (j == needle.size()) return i;
        }

        return -1;
    }
};

Java

public class Solution {
    public int strStr(String haystack, String needle) {
        if (haystack == null && needle == null) return 0;
        if (haystack == null) return -1;
        if (needle == null) return 0;
        
        for (int i = 0; i < haystack.length() - needle.length() + 1; i++) {
            int j = 0;
            for (; j < needle.length(); j++) {
                if (haystack.charAt(i+j) != needle.charAt(j)) break;
            }
            if (j == needle.length()) return i;
        }

        return -1;
    }
}

Source Code Analysis

1. corner case: haystack(source) and needle(target) may be empty string.
2. code convention:
   • space is needed for ==
   • use meaningful variable names
   • put a blank line before declaration int i, j;
3. declare j outside for loop if and only if you want to use it outside.

Some Pythonic notes: 4. More Control Flow Tools section 4.4 and if statement - Why does python use 'else' after for and while loops?

Complexity Analysis

nested for loop, $$O((n-m)m)$$ for worst case.