russian-doll-envelopes

https://leetcode.com/problems/russian-doll-envelopes/

// Use map (Russian doll number -> vector of envelopes) to record results
// For each envelope, check above map, and when fitting envelope which can hold current one is found,
// no need to check more envelope as later envelope is with smaller size or smaller Russian doll number.

bool lesser(const pair<int, int> &m1, const pair<int, int> &m2) {
    // sort function
    return m1.first < m2.first || (m1.first == m2.first && m1.second < m2.second);
}

class Solution {
    // Russian doll number -> vector of envelopes
    map<int, vector<pair<int, int>>> mp;
    map<int, vector<pair<int, int>>>::reverse_iterator itr;
    vector<pair<int, int>>::iterator vitr;
    // bool for whether best answer is reached
    bool fit;
public:
    int maxEnvelopes(vector<pair<int, int>>& envelopes) {
        sort(envelopes.begin(), envelopes.end(), lesser);
        int vlen = envelopes.size();
        if (vlen == 0) {
            return 0;
        }

        // loop from big envelope to small envelope
        for (int i=vlen-1; i>=0; i--) {
            // with the order of Russian doll, check whether current envelope can fit previous one
            for(itr = mp.rbegin(); itr != mp.rend(); ++itr) {
                fit = false;
                for (vitr = itr->second.begin(); vitr != itr->second.end(); ++vitr) {
                    if (envelopes[i].first < (*vitr).first &&
                        envelopes[i].second < (*vitr).second) {

                        // find fitting envelope, add one to Russian doll answer and record
                        if (mp.find(itr->first + 1) == mp.end()) {
                            vector<pair<int, int>> tmpvec;
                            tmpvec.push_back(envelopes[i]);
                            mp[itr->first + 1] = tmpvec;
                        }
                        else {
                            mp[itr->first + 1].push_back(envelopes[i]);
                        }
                        fit = true;
                        break;
                    }
                }
                if (fit) {
                    // if current envelope fit Russian doll, no need to check envelope with less answer
                    break;
                }
            }
            if (itr == mp.rend()) {
                // if no fitting envelope, current envelope is with Russian doll answer 1
                if (mp.find(1) == mp.end()) {
                    vector<pair<int, int>> tmpvec;
                    tmpvec.push_back(envelopes[i]);
                    mp[1] = tmpvec;
                }
                else {
                    mp[1].push_back(envelopes[i]);
                }
            }
        }

        // return the most Russian doll answer
        return mp.rbegin()->first;
    }
};