/**
 * @param {string} s
 * @return {number}
 */
var lengthOfLongestSubstring = function(s) {
    if(s.length < 2) return s.length;

    let seenChars = {};
    let startId = 0;
    let maxLength = 0;
    let currLength = 0;

    for(let i = 0; i < s.length; i++){
        const currChar = s[i];

        if(seenChars[currChar] !== undefined) {
            currLength = i - startId;
            if(currLength > maxLength) maxLength = currLength;
            startId = seenChars[currChar] + 1;
            seenChars[currChar] = i;

            const keys = Object.keys(seenChars);

            for(let j = 0; j < keys.length; j++){
                const seenCharValue = seenChars[keys[j]];
                if(seenCharValue < startId) seenChars[keys[j]] = undefined;
            }
        } else {
            seenChars[currChar] = i;
        }
    }

    if(s.length - startId > maxLength) return s.length - startId;
    return maxLength;
};