I need to find the substrings within my array. If I have an array: ["abc", "abcd", "abcde", "xyz"], my method should return the array members: abc, abcd, abcde as each is a substring or a superstring of the other, but it should exclude "xyz" as it is not related to other strings in any way.

function find_substrings(arr) {
    var res = [];
    for (var i=0; i<arr.length; i++) {
        for (var j=0; j<arr.length; j++) {
            if (i !== j && (arr[i].indexOf(arr[j]) > -1 || arr[j].indexOf(arr[i]) > -1)) {
    return res;
var arr = ["abc", "abcd", "abcde", "xyz"];

Here my code is of \$O(n^2)\$ complexity, as it's iterating twice over the complete array. Is there any optimal solution?

  • \$\begingroup\$ I'm not sure I'm clear on the requirements. Does every element in the returned array need to be a substring or superstring of every other element in the array, or just of at least one other element in the array? \$\endgroup\$
    – Thriggle
    Nov 23, 2016 at 16:09
  • 1
    \$\begingroup\$ every element of the array need not to be a substring or superstring of other elements but at lest one element of the array \$\endgroup\$
    – Mr.7
    Nov 24, 2016 at 8:20
  • 2
    \$\begingroup\$ What should be output when array contains more than one related strings e.g. ['abc', 'abcd', 'abcde', 'xyz', 'wxyz', 'vwxyz']? \$\endgroup\$
    – Tushar
    Mar 30, 2017 at 3:22
  • \$\begingroup\$ @Thriggle I agree, the requirements are too vague. And a single example is insufficient to clarify the problem statement... \$\endgroup\$ May 24, 2018 at 4:57

2 Answers 2


Interesting question,

  • find_substrings should really be findSubstrings
  • I dislike arr, and prefer list
  • If you sort the array by the length of strings, then you never have to check prior elements
  • If you kept a list of matches and non-matches, then you would never have to check matches again
  • I found the test case troubling, since you are looking for only 1 set, whereas the code will retrieve any number of sets
  • The complexity calculation becomes tricky here, I would think a worst case would be n*n/2, and best case n.

In the end, I came up with something like this:

function findSubstrings(list) {
  var out = [],
    match, rest, i, j;
  //Sort the array by the length of the string
  list.sort((a, b) => a.length - b.length);

  for (i = 0; i < list.length - 1; i++) {
    match = [];
    rest = [];
    for (j = i; j < list.length; j++) {
      if (list[j].indexOf(list[i]) > -1) {
      } else {
    //Did we find a set?
    if (match.length > 1) {
      out = out.concat(match);
      list = rest;
      i = 0;
      //Shortcut, should we still bother looking?
      if (rest.length == 0 || rest.length == 1) {
        return out;
  return out;
var testCase = ["abc", "abcd", "abcde", "xyz", "tom", "tomd"];


One quick improvement you can do is to initialize inner for loop variable from
j = i+1 instead of j = 0 and you won't need to check i !== j

for (var j=i+1; j<arr.length; j++)
if (arr[i].indexOf(arr[j]) > -1 || arr[j].indexOf(arr[i]) > -1)
  • \$\begingroup\$ Unfortunately, this change also changes the output of the algorithm. Example: for input ["abc", "ab"] you return ["abc"] while OP returns ["abc", "ab"]. \$\endgroup\$
    – le_m
    Apr 28, 2017 at 23:44
  • \$\begingroup\$ Indeed, you assume probably a sorted array? \$\endgroup\$
    – konijn
    Jun 27, 2017 at 22:39

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