hihocoder-Weekly223-Interval Coverage
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hihocoder-Weekly223-Interval Coverage
题目1 : Interval Coverage
描述
You are given N intervals [S1, T1], [S2, T2], [S3, T3], ... [SN, TN] and a range [X, Y]. Select minimum number of intervals to cover range [X, Y].
输入
The first line contains 3 integers N, X and Y. (1 <= N <= 100000, 1 <= X < Y <= 1000000)
The following N lines each contain 2 integers Si, Ti denoting an interval. (1 <= Si < Ti <= 1000000)
输出
Output the minimum number of intevals to cover range [X, Y] or -1 if it is impossible.
- 样例输入
-
5 1 5 1 2 1 3 2 4 3 5 4 5
- 样例输出
-
2
题解:
参考:http://hihocoder.com/discuss/question/5567
使用贪心算法。对所有的interval上的排序之后,根据interval进行尽可能向后的选择。
#include <cstdio> #include <cstring> #include <cstdlib> const int MAXN = 100000 + 10; struct Node { int x, y; }nd[MAXN]; int n, x, y; #define max(a, b) (a)>(b)?(a):(b) int cmp(const void *a, const void *b) { Node *aa = (Node *)a; Node *bb = (Node *)b; if(aa->x == bb->x) { return (aa->y - bb->y); } return aa->x - bb->x; } int main(){ scanf("%d %d %d", &n, &x, &y); int max_y = 0; for(int i=0; i<n; ++i) { scanf("%d %d", &nd[i].x, &nd[i].y); max_y = max(max_y, nd[i].y); } qsort(nd, n, sizeof(Node), cmp); if(n <= 0 || nd[0].x > x || max_y < y) { printf("-1 "); }else{ int tmp_y=0, cnt = 1, start = x, end = 0; for(int i=0; i<n; ++i) { if(nd[i].y < start) { continue; } if(nd[i].x <= start) { tmp_y = max(tmp_y, nd[i].y); }else if(nd[i].x > end) { break; }else{ start = tmp_y; tmp_y = nd[i].y; ++cnt; } end = max(end, nd[i].y); if(end >= y) { break; } } if(end >= y) { printf("%d ", cnt); }else{ printf("-1 "); } } return 0; }
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