Checking Subsequence

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Unit 4 Session 2 (Click for link to problem statements)

U-nderstand

Understand what the interviewer is asking for by using test cases and questions about the problem.

• How should the function handle empty strings for s and/or t?
  • If s is empty, the function should return True as an empty string is a subsequence of any string. If t is empty and s is not, the function should return False.
• What is the expected behavior if s is longer than t?
  • The function should return False, as a longer string cannot be a subsequence of a shorter one.

P-lan

Plan the solution with appropriate visualizations and pseudocode.

General Idea: Use two pointers to traverse strings s and t. Increment the pointer for s only when characters match in both strings, but always increment the pointer for t.

1) Initialize two pointers, `s_index` for string `s` and `t_index` for string `t`.
2) Traverse string `t` using the `t_index` pointer:
  a) If characters at both pointers match, increment the `s_index` pointer.
  b) Always increment the `t_index` pointer.
3) Check if `s_index` has reached the length of `s`, which means all characters of `s` were found in `t` in order.
4) Return True if the entire `s` is a subsequence of `t`, otherwise return False.

⚠️ Common Mistakes

• Forgetting to increment the t_index when characters do not match, which could lead to an infinite loop.
• Returning True prematurely without checking if all characters of s have been matched.

I-mplement

def is_subsequence_corrected(s, t):
    s_index, t_index = 0, 0
    while s_index < len(s) and t_index < len(t):
        if s[s_index] == t[t_index]:
            s_index += 1
        t_index += 1
    return s_index == len(s)