测试用例未通过，求排查Java字符串模式匹配代码问题

Let's break down the issues in your code and fix them to pass both test cases:

Key Problems in Your Current Code #

1. Incorrect loop termination: Your while (i <= text.length()) condition doesn't check if the pattern was actually found (index != -1). This leads to unnecessary iterations and potential invalid character access when index is -1.
2. Wrong position update for next search: The line i = i + text.indexOf(pattern, index) + pattern.length(); is flawed. When index is -1, text.indexOf(pattern, index) returns 0 (since searching from -1 starts at the beginning), causing an infinite loop or skipping valid overlapping matches.
3. No handling of overlapping patterns: For test case (a), the pattern "aba" overlaps in "abababa" (positions 0, 2, 4). Your code skips these overlaps because it jumps pattern.length() positions each time.

Fixed Code #

Here's the corrected version that addresses all these issues:

public static ArrayList<Character> getCharsThatFollowPattern(String text, String pattern) {
    ArrayList<Character> characters = new ArrayList<>();
    int patternLength = pattern.length();
    int currentIndex = text.indexOf(pattern);

    while (currentIndex != -1) {
        int nextCharPosition = currentIndex + patternLength;
        // Only add the character if the pattern isn't at the very end of the text
        if (nextCharPosition < text.length()) {
            characters.add(text.charAt(nextCharPosition));
        }
        // Move to the next character after the current match to allow overlapping patterns
        currentIndex = text.indexOf(pattern, currentIndex + 1);
    }

    return characters;
}

How This Fix Works #

• Proper loop termination: We only continue looping as long as currentIndex (the position of the pattern) is not -1 (meaning the pattern was found).
• Overlap support: By starting the next search at currentIndex + 1 instead of currentIndex + patternLength, we ensure we don't skip overlapping pattern matches (like the "aba" in "abababa").
• Safe character access: We check if nextCharPosition is less than the text length before accessing the character, avoiding out-of-bounds errors and ensuring we only add characters for non-ending pattern occurrences.

Testing the Fixed Code #

• Test Case (a): For "abababa" and "aba", the code finds matches at positions 0, 2, and 4. It adds the characters at positions 3 ('b') and 5 ('b'), returning [b, b] as expected.
• Test Case (b): For "abcabdabcab" and "ab", matches are at 0, 3, 6, 9. It adds characters at 2 ('c'), 5 ('d'), 8 ('c'), returning [c, d, c] as expected.

内容的提问来源于stack exchange，提问作者Auceris