September LeetCoding Challenge, Day 9: Compare Version Numbers

September 13, 2020

This is part of a series of posts about the September LeetCoding Challenge. Check the first post for more information.

The problem for September 9 is Compare Version Numbers. Given two version numbers version1 and version2, represented as strings, you want to return -1 if version1 is smaller than version2, 1 if version1 is larger than version2, and 0 if they’re equal. Versions are strings consisting of one or more revisions joined by a .. Each revision consists of digits only, but may contain leading zeros. Every revision contains at least one character, so it’s impossible for a substring of two . to exist. Comparing versions consists in comparing the integer value of its revisions in left-to-right order. If a version doesn’t specify a revision at a given index, then the revision should be treated as 0.

A solution for this problem consists in splitting the two provided strings in two sequences of numbers and do a pairwise comparison of them. Whenever a version number is missing a revision, assume 0 as its value. The following is an implementation of this strategy:

class Solution {
  int str_to_int(string str) {
    int ans;
    istringstream ss(str);
    ss >> ans;
    return ans;

  vector<int> split_version(string version) {
    vector<int> ans;
    size_t pos;
    while ((pos = version.find('.')) != string::npos) {
      ans.push_back(str_to_int(version.substr(0, pos)));
      version.erase(0, pos + 1);
    return ans;

  int compareVersion(string version1, string version2) {
    vector<int> v1 = split_version(version1);
    vector<int> v2 = split_version(version2);
    int N1 = v1.size(), N2 = v2.size(), i = 0;
    while (i < N1 || i < N2) {
      int p1 = i < N1 ? v1[i] : 0;
      int p2 = i < N2 ? v2[i] : 0;
      if (p1 < p2)
        return -1;
      if (p1 > p2)
        return 1;
    return 0;