2311. Longest Binary Subsequence Less Than or Equal to K

You are given a binary string s and a positive integer k.

Return the length of the longest subsequence of s that makes up a binary number less than or equal to k.

Note:

• The subsequence can contain leading zeroes.
• The empty string is considered to be equal to 0.
• A subsequence is a string that can be derived from another string by deleting some or no characters without changing the order of the remaining characters.

class Solution {
public:
    int longestSubsequence(string s, int k) {
        long long now = 0, m = 0, n = s.length(), res = 0;
        
        for(int i = n - 1; i >= 0; i--,m++) {
            if(s[i] == '0') res++;
            else if(m <= 30) {
                if(((1ll<<m) + now) <= k){
                    now += (1ll<<m);
                    res++;
                }
            }
        }

        return res;
    }
};