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Problem solution · Java

Strings Differ by One Character

Strings Differ by One Character: a Java 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
53 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 Strings Differ by One Character, 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

  • 53 lines of Java from the credited upstream file 1554-2.java.
  • The implementation visibly relies on sequence storage, hash lookup, ordered lookup.
  • 5 loop blocks 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 codeStrings Differ by One Character · JavaJava
Use this to learn the idea, then write your own version.
class Solution {  public boolean differByOne(String[] dict) {    final int m = dict[0].length();    int[] wordToHash = new int[dict.length];     for (int i = 0; i < dict.length; ++i)      wordToHash[i] = getHash(dict[i]);     // Compute the hash without each letter.    // e.g. hash of "abc" = 26^2 * 'a' + 26 * 'b' + 'c'    //   newHash of "a*c" = hash - 26 * 'b'    int coefficient = 1;    for (int j = m - 1; j >= 0; --j) {      Map<Integer, List<Integer>> newHashToIndices = new HashMap<>();      for (int i = 0; i < dict.length; ++i) {        final int newHash =            (int) ((wordToHash[i] - (long) coefficient * val(dict[i].charAt(j)) % HASH + HASH) %                   HASH);        if (newHashToIndices.containsKey(newHash)) {          for (final int index : newHashToIndices.get(newHash)) {            if (dict[i].substring(0, j).equals(dict[index].substring(0, j)) &&                dict[i].substring(j + 1).equals(dict[index].substring(j + 1))) {              return true;            }          }        }        newHashToIndices.putIfAbsent(newHash, new ArrayList<>());        newHashToIndices.get(newHash).add(i);      }      coefficient = (int) (coefficient * BASE % HASH);    }     return false;  }   private static final long BASE = 26;  private static final int HASH = 1_000_000_007;   private static int val(char c) {    return c - 'a';  }   // Returns the hash of `s`. Assume the length of `s` is m.  // e.g. getHash(s) = 26^(m - 1) * s[0] + 26^(m - 2) * s[1] + ... + s[m - 1].  private int getHash(String s) {    int hash = 0;    for (final char c : s.toCharArray()) {      hash = (int) ((hash * BASE + val(c)) % HASH);    }    return hash;  }}

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