4.区间覆盖 区间问题

 

       

#include <bits/stdc++.h>
using namespace std;
const int N = 100010;
struct Range {
    int l, r;
} range[N];
bool cmp(Range r1, Range r2) {
    return r1.l < r2.l;
}
int main() {
    int st, ed;
    cin >> st >> ed;
    int n;
    cin >> n;
    for (int i = 0; i < n; i++) {
        cin >> range[i].l >> range[i].r;
    }
    sort(range, range + n, cmp);
    bool success = false;
    int res = 0;
    for (int i = 0; i < n; i++) {
        //用双指针算法扫描一遍
        int j = i, r =-2e9;
        while (j < n && range[j].l <= st) {
            r = max(range[j].r,  r);
            j++;
        }
        /*
        上面扫描的作用就是找到：
        所有左端点在st的左边的所有区间里面
        右端点的最大值是多少
        */
        res++;
        if (r < st) {
            res = -1;
            break;
        }
        st = r;
        if (st >= ed) {
            success = true;
            break;
        }
        i = j - 1;
    }
    if (!success) {
        res = -1;
    }
    cout << res << endl;
    return 0;
}