Cracking the Coding Interview: Check Permutation

Photo by Jean-Louis Paulin

This post is part of the Algorithms Problem Solving series and a problem from the Cracking the Coding Interview book.

Problem description

Given two strings, write a method to decide if one is a permutation of the other

Examples

input: 'aba', 'aab'
output: true

input: 'aba', 'aaba'
output: false

input: 'aba', 'aa'
output: false

Solutions

If they have different lengths, they are not permutations.

Build a hashmap of characters of one of the strings Iterate through the other string and check if the characters are in the hashmap If there's no match in the hashmap, they are not permutations If it passes the loop, they are permutations.

• Runtime Complexity: O(N), where N = the length of the string
• Space Complexity: O(N), where N = the length of the string

function buildCharsMap(string) {
  const charsMap = new Map();

  for (let char of string) {
    if (charsMap.has(char)) charsMap.set(char, charsMap.get(char) + 1);
    else charsMap.set(char, 1);
  }

  return charsMap;
}

function checkPermutation(s1, s2) {
  if (s1.length !== s2.length) return false;
  const charsMap = buildCharsMap(s1);

  for (let char of s2) {
    if (!charsMap.has(char)) {
      return false;
    }

    if (charsMap.get(char) === 0) {
      return false;
    }

    charsMap.set(char, charsMap.get(char) - 1);
  }

  return true;
}

Resources

• Algorithms Problem Solving Series
• Algorithms & Data Structures studies
• Is Unique source code
• Data Structures in JavaScript Series
• Stack Data Structure in JavaScript
• Queue Data Structure in JavaScript
• Linked List Data Structure in JavaScript
• Tree Data Structure in JavaScript
• Stack Data Structure
• Queue Data Structure
• Linked List
• Tree Data Structure

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