给出 n 条线段（数轴上的整点线段），对于在其中选择 k 条的所有方案，求选中线段的交线段包含的整点数目的和。答案取模 1e9+7。



http://codeforces.com/problemset/problem/689/E



n 条线段相互交叉，形成了多个小线段。计算每个小线段被多少给出的线段包含。例如一个长 5 的小线段被 9 个线段包含，而题目要求选择 3 个区间，那麽这个小区间对答案的贡献就是 5 × pascal(9, 3)。



说起来容易，写起来还是值得学习的。一开始在下执着于离散化。但是后来学习别人的代码，才发现可以直接扫描线而不需离散化。-Given n line segments (the number line the whole point of the segment), where k is selected for all of the programs, the whole purpose of seeking points of the selected line segments cross and contained. The answer modulo 1e9+7. http://codeforces.com/problemset/problem/689/E n line segments intersect with each other, forming a plurality of small segments. Calculated for each segment is small given the number of segments contained. For example, a small length of line 5 is 9 segment contains the title and asked to　the three intervals, then the contribution of this inter-cell answers is 5 pascal (9, 3). Easy to say, it is still worth learning to write. Beginning next obsessed discrete. But then learn other people' s code, only to find you can directly scan line without discrete.