#425 - Word Squares

Hard • View on LeetCode • View on GitHub

Problem Description

Given an array of unique strings words, return all the word squares you can build from words. The same word from words can be used multiple times. You can return the answer in any order.

A sequence of strings forms a valid word square if the kth row and column read the same string, where 0 <= k < max(numRows, numColumns).

For example, the word sequence ["ball","area","lead","lady"] forms a word square because each word reads the same both horizontally and vertically.

Solution

/**
 * @param {string[]} words
 * @return {string[][]}
 */
var wordSquares = function(words) {
  const result = [];
  const prefixMap = new Map();
  const wordLength = words[0].length;

  for (const word of words) {
    for (let i = 0; i < word.length; i++) {
      const prefix = word.slice(0, i);
      if (!prefixMap.has(prefix)) {
        prefixMap.set(prefix, []);
      }
      prefixMap.get(prefix).push(word);
    }
  }

  function buildSquare(currentSquare) {
    if (currentSquare.length === wordLength) {
      result.push([...currentSquare]);
      return;
    }

    const prefix = currentSquare
      .map(word => word[currentSquare.length])
      .join('');

    const candidates = prefixMap.get(prefix) || [];
    for (const candidate of candidates) {
      currentSquare.push(candidate);
      buildSquare(currentSquare);
      currentSquare.pop();
    }
  }

  for (const word of words) {
    buildSquare([word]);
  }

  return result;
};