Interval merge [pair, sort]

Merge the intervals that have intersections (including endpoints)
802. Interval Sum - AcWing Question Bank

For example, now there is such a data

1 2
2 4 
5 6
7 8
7 9

The final result of the merger is

• 1 —— 4
• 5 —— 6
• 7 —— 9

get three intervals

Step 1: Sort by the left endpoint of the interval

Step 2: Scan the entire interval (during scanning, merge intervals)

Maintain the current range, e.g.

• left endpoint:st
• right endpoint:ed

There are several situations

within the range

In this case, both st and ed不需要变化

Intersect

At this time 修改ed, that is

no intersection

Because the left endpoint has been sorted from small to large, so the latter case will only be behind the purple point, and will not appear in the front

• Then put the st and ed at this point in the answer
• and update st and ed

resolve code

#include<iostream>
#include<algorithm>
#include<vector>

using namespace std;


typedef pair <int , int > pii;

vector<pii> nums;

void merge(vector<pii> &nums){
    
    
    vector<pii> res;
  
    int st = -2e9 , ed = -2e9;
  
    for(auto item : nums){
    
    
      
        if(item.first > ed){
    
    
            if(st != -2e9) res.push_back({
    
    item.first , item.second});
          
            st = item.first , ed = item.second;
          
        }else{
    
    
            ed = max(ed , item.second);
        }
      
    }
  
    if(st != -2e9){
    
    
        res.push_back({
    
    st , ed});
    }
  
    nums = res;
  
}

int main(){
    
    
    int n ;
  
    cin >> n;
  
    int l , r;
  
    for(int i = 0 ; i <n ; i ++){
    
    
        cin >> l >> r;
        nums.push_back({
    
    l , r});
    }
  
    sort(nums.begin() , nums.end());
  
    merge(nums);
  
    cout << nums.size();
  
    return 0;
}

Use pair to store left and right endpoints

• first: left endpoint
• second: right endpoint

typedef pair <int , int > pii;

vector<pii> nums;

Interval merge template

When sorting, it is the default sort first, when first is equal, sort second

void merge(vector<pii> &nums){
    
    
    vector<pii> res;
  
    int st = -2e9 , ed = -2e9;
  
    for(auto item : nums){
    
    
      
        if(item.first > ed){
    
    
            if(st != -2e9) res.push_back({
    
    item.first , item.second});
          
            st = item.first , ed = item.second;
          
        }else{
    
    
            ed = max(ed , item.second);
        }
      
    }
  
    if(st != -2e9){
    
    
        res.push_back({
    
    st , ed});
    }
  
    nums = res;
  
}

• Set the default value to 2e9, because the title defaults to 10 ⁹

• The current interval is strictly on the left of the maintenance interval

  ——(corresponding to the case of no intersection)

• in the interval

  (corresponding to intersection + interval)