4006. Count Valid Prefixes

You are given a binary string s.

A prefix of s is considered valid if its characters can be rearranged to form an alternating string.

Return the number of valid prefixes of s.

A string is considered alternating if no two adjacent characters are equal.

class Solution {
public:
    int countValidPrefixes(string s) {
        int res = 0, sum = 0;
        for(auto& ch : s) {
            sum += (ch == '0' ? -1 : 1);
            if(abs(sum) <= 1) res++;
        }
        return res;
    }
};