Clear Digits — C++

// Time:  O(n)
// Space: O(1)

// two pointers
class Solution {
public:
    string clearDigits(string s) {
        int j = 0;
        for (int i = 0; i < size(s); ++i) {
            if (isdigit(s[i])) {
                --j;
                continue;
            }
            s[j++] = s[i];
        }
        s.resize(j);
        return s;
    }
};

// Time:  O(n)
// Space: O(1)
// stack
class Solution2 {
public:
    string clearDigits(string s) {
        string result;
        for (const auto& x : s) {
            if (isdigit(x)) {
                result.pop_back();
                continue;
            }
            result.push_back(x);
        }
        return result;
    }
};