Is Subsequence

Given two lowercase strings, s and t, return true if s is a subsequence of t, or false otherwise. A subsequence of a string is a string that is formed from the original string by deleting some (can be none) of the characters while maintaining the relative positions of the remaining characters.

My solution was:

public class Solution {
  private string _s;
  private string _t;

  public bool IsSubsequence(string s, string t) {
    _s = s;
    _t = t;
    return IsReachable(0, 0);
  }

  private bool IsReachable(int si, int ti)
  {
    if (si == _s.Length)
      return true;

    char c = _s[si];
    for (int i = ti; i < _t.Length; i++)
    {
      if (_t[i] == c)
        return IsReachable(si + 1, i + 1);
    }
    return false;
  }
}

… where _s and _t avoid unnecessary string copies in the recursion tree.

public bool IsSubsequence(string s, string t) {
  return IsReachable(0, 0);

  bool IsReachable(int si, int ti) { ... }
}

’s iterative solution is much cleaner. Trying to implement it myself:

public bool IsSubsequence(string s, string t) {
  int si = 0, ti = 0;
  while (si < s.Length)
  {
    bool foundNextChar = false;
    for (int i = ti; i < t.Length; i++)
    {
      if (t[i] == s[si])
      {
        ti = i + 1;
        si += 1;
        foundNextChar = true;
        break;
      }
    }
    if (!foundNextChar)
      return false;
  }
  return si == s.Length;
}

… but I still have si, ti, and i lingering over. Why can I not come up with the simpler algorithm from ?

public bool IsSubsequence(string s, string t) {
  int si = 0, ti = 0;
  while (si < s.Length && ti < t.Length)
  {
    if (s[si] == t[ti])
      si += 1;

    ti += 1;
  }
  return si == s.Length;
}

I think I over-anchored too early on complicating this problem. I was already thinking that I need to consider every possible starting point of the subsequence. However, when looking for subsequence ac in a1b2a1c:

… there is no point in considering a at index \(4\) when we’ve already satisfied a from index \(0\).

1. Is Subsequence - LeetCode. leetcode.com . Accessed May 28, 2026.
2. Is Subsequence - LeetCode > Two Pointer Solution. leetcode.com . Accessed May 28, 2026.
3. Local functions - C# | Microsoft Learn. learn.microsoft.com . Accessed May 28, 2026.