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Leetcode 890. Find and Replace Pattern Sure, let’s break down the solution for the LeetCode problem “890. Find and Replace Pattern”. Here we will discuss the problem statement, potential clarifications, a detailed strategy, code implementation in C++, and analysis of the time complexity.

Problem Statement

You are given a list of strings words and a string pattern. You need to return a list of words that match the given pattern.

A word matches the pattern if there exists a permutation of letters such that pattern could be transformed into the word. Match the pattern means that there is a bijection between a letter in pattern and a letter in word.

Clarifying Questions

  1. Length of Words and Pattern: Do all the words in the words list and the pattern have the same length?
    • Clarification: Yes, all the strings are of the same length.
  2. Character Set: Are the words and pattern strictly lower-case English letters (a-z)?
    • Clarification: Yes, they are all lowercase English letters.

Strategy

To solve the problem, you need to compare the structure of each word with the pattern. This can be achieved using a mapping mechanism to ensure each character in the word maps to a character in the pattern uniquely and vice versa.

Here’s a step-by-step strategy:

  1. Normalization: Create a helper function to normalize both the word and the pattern. Transform the word/pattern such that characters are replaced by their first occurrence indices.
  2. Mapping Check: For the word and pattern to match, their normalized forms must be identical.
  3. Normalization Process: For example, “abb” would be normalized to “011”, because the first character is ‘a’ at index 0, the second character ‘b’ is new so gets 1, and the third character, also ‘b’, gets the same number 1.

Code

Here is the C++ implementation of the strategy outlined:

#include <vector>
#include <string>
#include <unordered_map>
using namespace std;

vector<int> normalize(const string& s) {
    unordered_map<char, int> char_to_index;
    vector<int> normalized;
    for (char c : s) {
        if (char_to_index.find(c) == char_to_index.end()) {
            char_to_index[c] = char_to_index.size();
        }
        normalized.push_back(char_to_index[c]);
    }
    return normalized;
}

vector<string> findAndReplacePattern(vector<string>& words, string pattern) {
    vector<string> result;
    vector<int> pattern_norm = normalize(pattern);
    
    for (const string& word : words) {
        if (normalize(word) == pattern_norm) {
            result.push_back(word);
        }
    }
    
    return result;
}

Time Complexity

This approach ensures a clear, concise, and efficient solution to identify and select words matching the given pattern in terms of structural character mapping.

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