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Problem solution · C++

String Transforms Into Another String

String Transforms Into Another String: a C++ solution using hash-based lookup. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Hash-based lookup
Source
walkccc LeetCode Solutions
Length
22 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Hash-based lookup

For String Transforms Into Another String, the implementation stores previously seen values or frequencies in a hash table for direct membership and lookup operations.

  1. Decide the key that represents the information needed later.
  2. Update its count or stored state while scanning the input.
  3. Use constant-time expected lookups to detect matches or assemble the result.

Code notes

  • 22 lines of C++ from the credited upstream file 1153.cpp.
  • The implementation visibly relies on sequence storage, hash lookup.
  • 1 loop block detected.

Complexity

Expected hash operations are constant time, but the surrounding scan and the number of stored keys determine total work and memory.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeString Transforms Into Another String · C++C++
Use this to learn the idea, then write your own version.
class Solution { public:  bool canConvert(string str1, string str2) {    if (str1 == str2)      return true;     vector<int> mappings(128);     for (int i = 0; i < str1.length(); ++i) {      const int a = str1[i];      const int b = str2[i];      if (mappings[a] != 0 && mappings[a] != b)        return false;      mappings[a] = b;    }     // No letter in the str1 maps to > 1 letter in the str2 and there is at    // lest one temporary letter can break any loops.    return unordered_set<char>(str2.begin(), str2.end()).size() < 26;  }};

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