[LeetCode] 68. Text Justification

The code is much dirtier than I thought, but I can't think of a way to make it okay.

class Solution {
public:
    vector<string> fullJustify(vector<string>& words, int maxWidth) {
        int ch_cnt = 0;
        vector<string> bag;
        vector<string> res;
        for (string& word: words) {
            if (ch_cnt + !bag.empty() + word.length() > maxWidth) {
                string build;
                int num_ws = bag.size() - 1;
                int tot_ws = maxWidth - (ch_cnt - num_ws);
                for (int i=0; i<num_ws; i++) {
                    build += bag[i];
                    build += string((tot_ws/num_ws) + (tot_ws%num_ws>i), ' ');
                }
                build += bag[num_ws];
                if (build.length() < maxWidth) {
                    build += string(maxWidth - build.length(), ' '); }
                res.push_back(build);
                bag.clear(); ch_cnt = 0;
            }
            ch_cnt += word.length() + !bag.empty();
            bag.push_back(word);
        }
        if (!bag.empty()) {
            string build;
            int num_ws = bag.size() - 1;
            for(int i=0; i<num_ws; i++) {
                build += bag[i]; build += " ";
            }
            build += bag[num_ws];
            if (build.length() < maxWidth) {
                build += string(maxWidth - build.length(), ' '); }
            res.push_back(build);
        }
        return res;
    }
};