I'm trying to solve this using the .every method but it's not returning true and therefore it's not adding onto my string and I'm not sure why.

var longestCommonPrefix = function(arr) {
  if (arr.length === 0) {
    return undefined;
  }
  let result = '';
  for (let i = 0; i < arr.length; i++) {
    if (arr.every(x => arr[i].charAt(i) === x)) {
      result += arr[i].charAt(i);
    }
  }
  return result
}

console.log(longestCommonPrefix(["flower", "flow", "flight"])); //fl

I'm trying to solve this using the .every method but it's not returning true and therefore it's not adding onto my string and I'm not sure why.

var longestCommonPrefix = function(arr) {
  if (arr.length === 0) {
    return undefined;
  }
  let result = '';
  for (let i = 0; i < arr.length; i++) {
    if (arr.every(x => arr[i].charAt(i) === x)) {
      result += arr[i].charAt(i);
    }
  }
  return result
}

console.log(longestCommonPrefix(["flower", "flow", "flight"])); //fl

• javascript

Share Improve this question Follow edited Apr 12, 2021 at 23:01 custommander 19.1k66 gold badges6969 silver badges9393 bronze badges asked Apr 12, 2021 at 21:19 Juan GomezJuan Gomez 1933 bronze badges

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4 Answers 4

Sorted by: Reset to default 6

You need to iterate over one string, not over the whole array: check if the first character of the string is present everywhere, then the second character, etc:

var longestCommonPrefix = function(arr) {
  if (arr.length === 0) {
    return undefined;
  }
  let result = '';
  for (let i = 0; i < arr[0].length; i++) {
    if (arr.every(x => x.charAt(i) === arr[0][i])) {
      result += arr[i].charAt(i);
    } else break;
  }
  return result;
}

console.log(longestCommonPrefix(["flower", "flow", "flight"])); //fl

Your use of Array.every is along the right lines. You want to check that every string in the array has the same character at position i. I think you got confused when you named the parameter x, when it is in fact a string :)

var longestCommonPrefix = function(words) {
  if (words.length === 0) {
    return "";
  }
  
  const letters = [];
  const lengthOfShortestWord = Math.min(...words.map(word => word.length));
  
  for (let i = 0; i < lengthOfShortestWord; i++) {
    const char = words[0][i];
    if (words.every(word => word[i] === char)) {
      letters.push(char);
    } else {
      break;
    }
  }
  return letters.join("");
}

console.log(longestCommonPrefix(["flower", "flow", "flight"])); //fl

Unless I am mistaken the longest prefix is never going to be greater than the smallest string in the array.

In this case "fl" is both the smallest string and the longest mon prefix:

["flower", "fl", "flight"]

So start with finding the smallest string in arr:

let [sm] = [...arr].sort((a, b) => a.length - b.length);

Then check that all strings in arr start with sm:

arr.every(str => str.startsWith(sm));

If that isn't the case then shorten sm by one character:

sm = sm.slice(0, -1);

And keep going until you eventually find the longest prefix or sm bees an empty string:

const prefix = arr => {
  let [sm] = [...arr].sort((a, b) => a.length - b.length);
  while (sm && !arr.every(str => str.startsWith(sm))) sm = sm.slice(0, -1);
  return sm;
};

The most important is what @CertainPerformance said: "You need to iterate over one string" and with this logic you can use something like this:

var longestCommonPrefix = (strs) => {
    if (strs.length === 0) return '';

    let lettersInCommon = '';

    for (let i = 0; i < strs[0].length; i++) {
        if (strs.every(str => str.charAt(i) === strs[0][i])) {
            lettersInCommon += strs[0][i];
        } else {
            break;
        }
    }

    return lettersInCommon;
};

console.log(longestCommonPrefix(["flower","flow","flight"]))