def maxLength(arr):
    dp = ['']
    maxLen = 0

    for s in arr:
        if len(set(s)) < len(s): continue
        for i in range(len(dp) - 1, -1, -1):
            tmp = dp[i] + s
            if len(set(tmp)) == len(tmp):
                dp.append(tmp)
                maxLen = max(maxLen, len(tmp))

    return maxLen

The algorithm initializes a list called 'dp' containing an empty string. Then, for each string s in the input array 'arr', it checks whether the string has unique characters by comparing its length with the length of a set that contains its individual characters.

If the string has unique characters, it iterates through the dp list in reverse order, forming a new string by concatenating the current string from the dp list and s. The algorithm checks if this new concatenated string has unique characters. If it does, it appends the concatenated string to the dp list and updates the maximum length of unique characters seen so far.

By the end of this process, the algorithm returns the maximum length of unique characters in a concatenated string from the subsequence of arr.

#include <vector>
#include <string>
#include <algorithm>

int maxLength(std::vector<std::string>& arr) {
    std::vector<std::string> dp = {""};

    int maxLen = 0;
    for (std::string s : arr) {
        if (std::unordered_set<char>(s.begin(), s.end()).size() < s.size()) continue;
        for (int i = dp.size() - 1; i >= 0; --i) {
            std::string tmp = dp[i] + s;
            if (std::unordered_set<char>(tmp.begin(), tmp.end()).size() == tmp.size()) {
                dp.push_back(tmp);
                maxLen = std::max(maxLen, (int)tmp.size());
            }
        }
    }

    return maxLen;
}

The algorithm initializes a list called 'dp' containing an empty string. Then, for each string s in the input array 'arr', it checks whether the string has unique characters by comparing its length with the length of a set that contains its individual characters.

If the string has unique characters, it iterates through the dp list in reverse order, forming a new string by concatenating the current string from the dp list and s. The algorithm checks if this new concatenated string has unique characters. If it does, it appends the concatenated string to the dp list and updates the maximum length of unique characters seen so far.

By the end of this process, the algorithm returns the maximum length of unique characters in a concatenated string from the subsequence of arr.