longestNonrepeatingSubstring('abcabcbb'); // abc

longestNonrepeatingSubstring('aaaaaa'); // a

longestNonrepeatingSubstring('ababbab'); // ab

longestNonrepeatingSubstring('abcda'); // abcd

longestNonrepeatingSubstring('abba'); // ab

longestNonrepeatingSubstring('pwwkew'); // wke

longestNonrepeatingSubstring('dvdf'); // vdf

longestNonrepeatingSubstring('xyzzabcd'); // zabcd

function longestNonrepeatingSubstring(s) {

let start = 0;

let maxSubstring = '';

let charIndex = {};

for (let i = 0; i < s.length; i++) {

if (charIndex[s[i]] >=

) {

start = charIndex[s[i]] + 1;

}

charIndex[s[i]] = i;

if (

> maxSubstring.length) {

maxSubstring = s.

(start, i + 1);

}

}

return maxSubstring;

}