Meeting Time, C++ (gcc)

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/*
#17. Meeting Time 
Input is a number of meetings (start_time, end_time)。 Output is a number of time intervals (start_time, end_time), where there is no meetings.
For exmaple, input is [[1, 3], [6, 7]], [[2, 4]], [[2, 3], [9, 12]] ]
Output [[4, 6], [7, 9]]
Leetcode 相似: Leetcode 252/253 Meeting Rooms I/II;  759. Employee Free Time
*/

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

struct Interval{
    int start;
    int end; 
    Interval(int s, int e): start(s), end(e) {}
};

void print(vector<Interval>& v){
    cout << "[";
    for(int i =0; i<(int)v.size(); i++) 
        cout << "[" << v[i].start << "," << v[i].end << "]"; 
    cout << "]" << endl; 
}

bool compare(Interval& a, Interval& b){
    return a.start < b.start; 
}

vector<Interval> getTime(vector<vector<Interval>>& v){
    vector<Interval> res; 
    vector<Interval> all; 
    for(auto x: v){
        all.insert(all.end(), x.begin(), x.end()); 
    }
    sort(all.begin(), all.end(), compare);
    print(all);
  
    int pre = all[0].end;
    for(int i=1; i<(int)all.size(); i++){
        if(all[i].start > pre) // found one gap
        {
            res.push_back(Interval(pre, all[i].start));
            pre = all[i].end;
        }
        else{
            pre = max(pre, all[i].end);
        }
    }
    return res; 
}

int main() {
    vector<vector<Interval>> vs = {{Interval(0,1), Interval(2,3)}, {Interval(3,4),Interval(5,6)}};  
    vector<Interval> res = getTime(vs);
    print(res);
    return 0; 
}

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Compilation time: 1.03 sec, absolute running time: 0.07 sec, cpu time: 0.01 sec, memory peak: 3 Mb, absolute service time: 1,1 sec