696. 计数二进制子串

class Solution {
    public int countBinarySubstrings(String s) {
        int n = s.length();
        int res = 0;
        int pre = 0, cur = 1;
        for(int i = 1; i < n; i++) {
            if(s.charAt(i) == s.charAt(i-1)) cur++;
            else {
                pre = cur;
                cur = 1;
            }
            if(cur <= pre) res++;
        }
        return res;
    }
}