C++ 解法, 执行用时: 3ms, 内存消耗: 484KB, 提交时间: 2020-07-28

#include <bits/stdc++.h>
  
using namespace std;
  
vector <pair<int, int> > pv;
  
bool dfs(int index, int x, int y){
    if (index == pv.size()){
        return true;
    }
    if (x >= y){
        return false;
    }
    if(x >= pv[index].first && y <= pv[index].second){
        return false;
    }
    if(y <= pv[index].first || x >= pv[index].second){
        return dfs(index + 1, x, y);
    }
    if(x >= pv[index].first && y >= pv[index].second){
        return dfs(index + 1, pv[index].second, y);
    }
    if(x <= pv[index].first && y <= pv[index].second){
        return dfs(index + 1, x, pv[index].first);
    }
    if(x <= pv[index].first && y >= pv[index].second){
        return dfs(index + 1, x, pv[index].first) || dfs(index + 1, pv[index].second, y);
    }
     
    return false;
  
}
  
int main() {
    int n, res = 0;
    scanf("%d", &n);
    for (int i = 0; i < n; i++){
        pair<int, int> p;
        scanf("%d %d", &p.first, &p.second);
        pv.push_back(p);
    }
    for (int i = 0; i < n; i++){
        if (dfs(i + 1, pv[i].first, pv[i].second)){
            res++;
        }
    }
    printf("%d\n", res);
    return 0;
}

C++ 解法, 执行用时: 4ms, 内存消耗: 380KB, 提交时间: 2020-10-29

#include <bits/stdc++.h>
  
using namespace std;
  
vector <pair<int, int> > pv;
  
bool dfs(int index, int x, int y){
    if (index == pv.size()){
        return true;
    }
    if (x >= y){
        return false;
    }
    if(x >= pv[index].first && y <= pv[index].second){
        return false;
    }
    if(y <= pv[index].first || x >= pv[index].second){
        return dfs(index + 1, x, y);
    }
    if(x >= pv[index].first && y >= pv[index].second){
        return dfs(index + 1, pv[index].second, y);
    }
    if(x <= pv[index].first && y <= pv[index].second){
        return dfs(index + 1, x, pv[index].first);
    }
    if(x <= pv[index].first && y >= pv[index].second){
        return dfs(index + 1, x, pv[index].first) || dfs(index + 1, pv[index].second, y);
    }
     
    return false;
  
}
  
int main() {
    int n, res = 0;
    scanf("%d", &n);
    for (int i = 0; i < n; i++){
        pair<int, int> p;
        scanf("%d %d", &p.first, &p.second);
        pv.push_back(p);
    }
    for (int i = 0; i < n; i++){
        if (dfs(i + 1, pv[i].first, pv[i].second)){
            res++;
        }
    }
    printf("%d\n", res);
    return 0;
}