口算难题

import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;
import java.util.stream.IntStream;

public class VerbalArithmeticStream {
    // Main function to check if the equation is solvable using Streams
    public boolean isSolvable(String[] words, String result) {
        Set<Character> uniqueChars = new HashSet<>();
        for (String word : words) {
            word.chars().mapToObj(c -> (char) c).forEach(uniqueChars::add);
        }
        result.chars().mapToObj(c -> (char) c).forEach(uniqueChars::add);
        if (uniqueChars.size() > 10) {
            return false;
        }
        char[] chars = new char[uniqueChars.size()];
        int index = 0;
        for (char c : uniqueChars) {
            chars[index++] = c;
        }
        return backtrack(words, result, new HashMap<>(), new boolean[10], chars, 0);
    }

    private boolean backtrack(String[] words, String result, Map<Character, Integer> charToDigit, boolean[] usedDigits, char[] chars, int pos) {
        if (pos == chars.length) {
            return isValid(words, result, charToDigit);
        }
        return IntStream.range(0, 10).filter(d -> !usedDigits[d]).anyMatch(d -> {
            charToDigit.put(chars[pos], d);
            usedDigits[d] = true;
            boolean res = backtrack(words, result, charToDigit, usedDigits, chars, pos + 1);
            usedDigits[d] = false;
            charToDigit.remove(chars[pos]);
            return res;
        });
    }

    private boolean isValid(String[] words, String result, Map<Character, Integer> charToDigit) {
        int sum = IntStream.range(0, words.length).map(i -> getNumericValue(words[i], charToDigit)).sum();
        return sum == getNumericValue(result, charToDigit);
    }

    private int getNumericValue(String word, Map<Character, Integer> charToDigit) {
        return word.chars().map(c -> charToDigit.get((char) c)).reduce(0, (a, b) -> a * 10 + b);
    }
}