双指针

#include <bits/stdc++.h>
using namespace std;

unordered_map<char,int> um;
int maxn,minl;

int main()
{
    string s;
    cin >> s;
    int l=0,r=0;
    while(l<=r && r<s.length())
    {
        if(um[s[r]])
        {
            um[s[l]]--;
            l++;
        }
        else
        {
            um[s[r]]++;
            r++;
        }
        if(r-l>maxn)
        {
            maxn = r-l;
            minl = l;
        }
    }
    for(int i = minl;i<minl+maxn;i++)
        cout << s[i];
    return 0;
}

s = input()
maxn,minl = 0,0
dic = {}
l,r = 0,0
for i in s:
    dic[i] = 0
while l<=r and r<len(s):
    if dic[s[r]]:
        dic[s[l]]-=1
        l+=1
    else:
        dic[s[r]]+=1
        r+=1
    if r-l>maxn:
        maxn = r-l
        minl = l

print(s[minl:minl+maxn])