Isomorphic Strings Problem & Solution

Given two strings s and t, determine if they are isomorphic. Two strings s and t are isomorphic if the characters in s can be replaced to get t. All occurrences of a character must be replaced with another character while preserving the order of characters. No two characters may map to the same character, but a character may map to itself.

See the isomorphic strings problem on LeetCode.

C++ Solution

#pragma GCC optimize("Ofast")
#pragma GCC optimization("unroll-loops")

static const int _=[](){std::ios::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);return 0;}();

class Solution {
  bool isIsomorphic(string s, string t) {
    unordered_map<char, char> map;
    set<char> used;

    for (int i = 0; i < s.size(); ++i) {
      if (map.find(s[i]) == map.end() &&
          used.find(t[i]) == used.end()) {
        map[s[i]] = t[i];

      if (map[s[i]] != t[i]) {
        return false;

    return true;

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