I am working on a programming interview practice question in Python 3 (on codefights.com). The challenge is to return the length of the longest substring that appears in both of two input strings. The code must run in under 4 seconds for all tests, which include a few where the length of the two strings sum to 20,000.

As a shorter example:

longestCommonSubstring("zxabcdezy", "yzabcdezx") -> 6

What can I do to improve my code's runtime?

from math import inf

def longestCommonSubstring(s, t):
    """Return the length of the longest substring that appears in both s and t.
    This function builds a suffix tree of a combination of s and t using
    Ukkonen's algorithm. It assumes that the symbols $ and # appear in neither s
    nor t.

    len_s = len(s)
    string = s + '#' + t + '$'
    len_string = len(string)
    max_len = 0

    class LeafNode():

        def __init__(self, from_first_word):
            self.from_first_word = from_first_word

        def has_s_leaves(self):
            return self.from_first_word

        def has_t_leaves(self):
            return not self.from_first_word

    class InternalNode():

        def __init__(self, root_length):
            self.edges = {}  # dictonary of edges keyed by first letter of edge
            self.link = None
            self.root_length = root_length
            self.has_s_leaves = False
            self.has_t_leaves = False
            self.already_counted = False

        def __getitem__(self, key):
            return self.edges[key]

        def __setitem__(self, key, edge):
            self.edges[key] = edge
            # Update leaf identity based on new child leaves
            # Using "or" is faster than "|=" (I guess |= doesn't short circuit)
            self.has_s_leaves = self.has_s_leaves or edge.dest.has_s_leaves
            self.has_t_leaves = self.has_t_leaves or edge.dest.has_t_leaves

        def __contains__(self, key):
            return key in self.edges

    class Edge():

        def __init__(self, dest, start, end):
            self.dest = dest
            self.start = start
            self.end = end
            self.length = self.end - self.start

    root = InternalNode(0)

    class Cursor():

        def __init__(self):
            self.node = root
            self.edge = None
            self.idx = 0
            self.lag = -1

        def is_followed_by(self, letter):
            if self.idx == 0:
                return letter in self.node
            return letter == string[self.node[self.edge].start + self.idx]

        def defer(self, letter):
            """When we defer the insertion of a letter,
            we need to advance the cursor one position.
            self.idx += 1
            # We never want to leave the cursor at the end of an explicit edge.
            # If this is the case, move it to the beginning of the next edge.
            if self.edge is None:
                self.edge = letter
            edge = self.node[self.edge]
            if self.idx == edge.length:
                self.node = edge.dest
                self.edge = None
                self.idx = 0

        def post_insert(self, i):
            """When we are finished inserting a letter, we can pop
            it off the front of our queue and prepare the cursor for the
            next letter.
            self.lag -= 1
            # Only when the current node is the root is the first letter (which
            # we must remove) part of the cursor edge and index. Otherwise it is
            # implicitly determined by the current node.
            if self.node is root:
                if self.idx > 1:
                    self.edge = string[i - self.lag]
                    self.idx -= 1
                    self.idx = 0
                    self.edge = None
            # Following an insert, we move the active node to the node
            # linked from our current active_node or root if there is none.
            self.node = self.node.link if self.node.link else root
            # When following a suffix link, even to root, it is possible to
            # end up with a cursor index that points past the end of the current
            # edge. When that happens, follow the edges to a valid cursor
            # position. Note that self.idx might be zero and self.edge None.
            while self.edge and self.idx >= self.node[self.edge].length:
                edge = self.node[self.edge]
                self.node = edge.dest
                if self.idx == edge.length:
                    self.idx = 0
                    self.edge = None
                    self.idx -= edge.length
                    self.edge = string[i - self.lag + self.node.root_length]

        def split_edge(self):
            edge = self.node[self.edge]
            # Create a new node and edge
            middle_node = InternalNode(self.node.root_length + self.idx)
            midpoint = edge.start + self.idx
            next_edge = Edge(edge.dest, midpoint, edge.end)
            middle_node[string[midpoint]] = next_edge
            # Update the current edge to end at the new node
            edge.dest = middle_node
            edge.end = midpoint
            edge.length = midpoint - edge.start
            return middle_node

    cursor = Cursor()
    from_first_word = True
    dummy = InternalNode(0)

    for i, letter in enumerate(string):

        if from_first_word and i > len_s:
            from_first_word = False

        cursor.lag += 1
        prev = dummy  # dummy node to make suffix linking easier the first time

        while cursor.lag >= 0:

            if cursor.is_followed_by(letter):  # Suffix already exists in tree
                prev.link = cursor.node

            elif cursor.idx != 0:  # We are part-way along an edge
                stem = cursor.split_edge()
                stem = cursor.node
            # Now we have an explicit node and can insert our new edge there.
            stem[letter] = Edge(LeafNode(from_first_word), i, inf)
            # Whenever we update an internal node, we check for a new max_len
            # But not until we have started into the second input string
            if (i > len_s and not stem.already_counted
                and stem.has_s_leaves and stem.has_t_leaves):
                stem.already_counted = True
                if stem.root_length > max_len:
                    max_len = stem.root_length
            # Link the previously altered internal node to the new node and make
            # the new node prev.
            prev.link = prev = stem

    return max_len

For sake of comparison, I tested this algorithm against the dynamic programming approach (based on @Ludisposed answer).

Ukkonen's algorithm:

s, t = 1000 * "zxabcdezy", 1000 * "yzabcdezx"
%timeit longestCommonSubstring(s, t)
318 ms ± 12.1 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

Dynamic programming:

def longestCommonSubstring(S, T):
    if len(S) < len(T):
        S, T = T, S
    row = [0] * (len(T) + 1)
    longest = 0
    for i, si in enumerate(S):
        for j, tj in enumerate(reversed(T)):
            k = len(T) - j
            if si == tj:
                row[k] = row[k-1] + 1
                row[k] = 0
        longest = max(longest, *row)
    return longest

s, t = 1000 * "zxabcdezy", 1000 * "yzabcdezx"
%timeit longestCommonSubstring(s, t)
27.7 s ± 155 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
  • 2
    \$\begingroup\$ Should not the answer be 6 for the "zxabcdezy" and "yzabcdezx" strings? \$\endgroup\$
    – alecxe
    Commented Dec 27, 2017 at 3:00
  • 1
    \$\begingroup\$ By the way, longestCommonSubstring(1000 * "zxabcdezy", 1000 * "yzabcdezx") results into a RecursionError: maximum recursion depth exceeded error in the max_common_length() function. Are you sure your solution passes all the tests on codefights? Thanks. \$\endgroup\$
    – alecxe
    Commented Dec 27, 2017 at 3:06
  • 2
    \$\begingroup\$ @alecxe I ran the code in repl.it with the example provided, and it returned 6 rather than 4, I corrected the post's example case accordingly, must have been a typo. \$\endgroup\$
    – Phrancis
    Commented Dec 27, 2017 at 4:37
  • \$\begingroup\$ @Phrancis, thanks for catching my typo. It was supposed to say 6. \$\endgroup\$ Commented Dec 27, 2017 at 17:49
  • \$\begingroup\$ @alexce, None of the code fights test cases involved that much repetition (even the long strings). The repetition causes a very deep tree and walking that tree recursively caused the error you found. I changed the code to keep tabs on the max length as it builds the tree rather than walk it recursively after. \$\endgroup\$ Commented Dec 27, 2017 at 17:51

1 Answer 1



You can use the cProfiler to check where your program takes up the most amount of time

Example Code

import cProfile
pr = cProfile.Profile()

# Your function call
longestCommonSubstring(s, t)


DP alternative

There is another yet slower solution, with dynamic programming. However this would take less space but more time \$O(n*m)\$ where the Suffix tree approach is \$O(n + m)\$

See the wiki of the Longest Common Substring problem

def longestCommonSubstring(S, T):
    L = [[0]*(len(T)+1) for x in range(len(S)+1)]
    longest = 0

    for i in range(len(S)):
        for j in range(len(T)):
            if S[i] == T[j]:
                if i == 0 or j == 0:
                    L[i][j] = 1
                    L[i][j] = L[i-1][j-1] + 1

                if L[i][j] > longest:
                    longest = L[i][j]
                L[i][j] = 0
    return longest

NOTE that: This solution will not pass all the tests in time.


Under construction

  • \$\begingroup\$ (The title mentions Ukkonen, I can't seem to recognise it on codefights.com.) \$\endgroup\$
    – greybeard
    Commented Dec 27, 2017 at 11:31
  • \$\begingroup\$ @greybeard codefights.com/interview/Gh37HrvDBrqfLi2iA Challenge link, no mention of Ukkonen \$\endgroup\$
    – Ludisposed
    Commented Dec 27, 2017 at 16:23
  • \$\begingroup\$ The dynamic programming approach is too slow. Even with some optimizations to your code that reduce the memory requirements it runs much more slowly than the suffix tree approach (for very long strings). \$\endgroup\$ Commented Dec 27, 2017 at 17:58
  • \$\begingroup\$ @RobertPerrotta I see on CF that you have solved this within the time constraints?, feel free to post an answer under your question :) \$\endgroup\$
    – Ludisposed
    Commented Dec 28, 2017 at 14:19
  • \$\begingroup\$ @Ludisposed, I finally did solve the challenge! But I wasn't able to optimize my suffix tree method enough to meet the time constraints. Instead, I used a rolling hash/binary search method suggested by some other codefights users. I don't mind sharing my code, but it doesn't really answer the question I asked: how can I make this code run faster? Does it belong here in an answer anyway? \$\endgroup\$ Commented Dec 29, 2017 at 22:18

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