Leetcode Permutation in String problem 567

leetcode-567.js

/*
    Given two strings s1 and s2, return true if s2 contains a permutation of s1, or false otherwise.

    In other words, return true if one of s1's permutations is the substring of s2.
*/

const checkInclusion = (s1, s2) => {
    if (s1.length > s2.length) return false;

    const s1Dict = {};
    const s2Dict = {};

    // Create dictionary of s1
    for (let i = 0; i <= s1.length - 1; i++) {
        let letter = s1[i];
        let letter2 = s2[i];

        s1Dict[letter] = letter in s1Dict ? s1Dict[letter] + 1 : 1;
        s2Dict[letter2] = letter2 in s2Dict ? s2Dict[letter2] + 1 : 1;
    }

    if (objectsAreEqual(s1Dict, s2Dict)) return true;

    for (let i = s1.length; i <= s2.length - 1; i++) {
        // Remove letter from s2
        let letter = s2[i - s1.length];
        s2Dict[letter]--;

        let newLetter = s2[i];
        s2Dict[newLetter] = newLetter in s2Dict ? s2Dict[newLetter] + 1 : 1;
        if (objectsAreEqual(s1Dict, s2Dict)) return true;
    }

    return false;
};

const objectsAreEqual = (a, b) => {
    const keys = Object.keys(a);

    for (let i = 0; i <= keys.length - 1; i++) {
        let key = keys[i];

        if (a[key] !== b[key]) return false;
    }

    return true;
};