leetcode 第三题, 如下解法的时间复杂度是多少?

原题链接 https://leetcode.com/problems/longest-substring-without-repeating-characters/

是 O(n^2) 吗?

const lengthOfLongestSubstring = (s) => {
    let i = 0;
    let maxLen = 0;
    let tempStr = ''
    while (i < s.length) {
        const letter = s[i];
        const lastSameI = tempStr.indexOf(letter) 
        if (lastSameI >= 0) {
            const tempStrLen = tempStr.length;
            if ( tempStrLen > maxLen) {
                maxLen = tempStrLen;
            }
            tempStr = tempStr.slice(lastSameI + 1);
        } 
        tempStr += letter;
        i += 1;
    }
    const tempStrLen = tempStr.length;
    if ( tempStrLen > maxLen) {
        maxLen = tempStrLen;
    }
    return maxLen;
};