[AcWing 907] 区间覆盖

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点击查看代码

#include<iostream>
#include<algorithm>


using namespace std;

const int N = 1e5 + 10;

int st, ed, n, res;

struct Range {
	int l, r;
	bool operator < (const Range &W) const {return l < W.l;}
}range[N];

int main()
{
	cin >> st >> ed >> n;
	for (int i = 0; i < n; i ++) {
		int l, r;
		cin >> l >> r;
		range[i] = {l, r};
	}
	sort(range, range + n);
	bool ac = false;
	for (int i = 0; i < n; i ++) {
		int j = i, r = -2e9;
		while (j < n && range[j].l <= st) {
			r = max(r, range[j].r);
			j ++;
		}
		if (r < st) {
			res = -1;
			break;
		}
		res ++;
		if (r >= ed) {
			ac = true;
			break;
		}
		st = r;
		i = j - 1;
	}
	if (!ac)	res = -1;
	cout << res << endl;
	return 0;
}

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1. 算法思路
   ① 将所有区间按左端点从小到大排序
   ② 从前往后枚举每个区间，在所有能覆盖 $start$ 的区间中，选择右端点最大的区间，然后将 $start$ 更新成右端点的最大值