Accepted clean Java solution with explanation (two pointers)

  • 15

    The basic idea is we keep comparing s and t from the beginning, once there's a difference, we try to replace s(i) with t(j) or insert t(j) to s(i) and see if the rest are the same.

    Example: i and j are the two pointers of S and T, we found that 'b' != 'c' and we try to replace it:

         i                           i
    S: a c d      replace       S: a b d
    T: a b c d   --------->     T: a b c d    --->  "d" != "cd", no good
         j                           j

    now we try to insert T(j) to S(i) and we get:

         i                           i
    S: a c d      insert        S: a b c d
    T: a b c d   --------->     T: a b c d    --->  "cd" == "cd", viola!
         j                           j

    To keep the code simple, we make s is always shorter than t, so we don't need to try deletion.


    public boolean isOneEditDistance(String s, String t) {
      if (s == null || t == null)
        return false;
      if (s.length() > t.length())
        return isOneEditDistance(t, s);
      int i = 0, j = 0;
      while (i < s.length() && j < t.length()) {
        if (s.charAt(i) != t.charAt(j)) {
          // we try to replace s[i] with s[j] or insert s[j] to s[i]
          // then compare the rest and see if they are the same
          return s.substring(i + 1).equals(t.substring(j + 1)) ||
                 s.substring(i).equals(t.substring(j + 1));
        i++; j++;
      return t.length() - j == 1;

  • 0

    Nice solution, I rewrite it in Python language:

    def isOneEditDistance(self, s, t):
        m, n = len(s), len(t)
        if m > n:
            return self.isOneEditDistance(t, s)
        if n-m > 1:
            return False
        i, j = 0, 0
        while i < m and j < n:
            if s[i] != t[j]:
                return s[i+1:] == t[j+1:] or s[i:] == t[j+1:]
            i += 1; j += 1
        return n-m == 1

  • 4

    Excellent idea!
    But I think we have no need to maintain index j on String t, since i and j are always in the in the same pace until we return. I have implemented your idea without using index j, and it was accepted.

    public boolean isOneEditDistance(String s, String t) {
        int s_len = s.length();
        int t_len = t.length();
        if (t_len < s_len)
            return isOneEditDistance(t, s);
        if (t_len - s_len > 1)
            return false;
        int i = 0;
        while (i < s_len) {
            if (s.charAt(i) != t.charAt(i))
                return s.substring(i+1).equals(t.substring(i+1)) || s.substring(i).equals(t.substring(i+1));
        return s_len + 1 == t_len;

  • 0

    Yeah you are right!

  • 0

    Doesn't the substring() operation in Java 7 and higher consume a O(N) complexity?

  • 0

    Thanks mate!

  • 0
    This post is deleted!

  • 0
      return t.length() - j == 1;

    Could you explain why you have this statement please? Thanks.

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