CF 581 Div2. E. Natasha, Sasha and the Prefix Sums
https://codeforces.com/contest/1204/problem/E
I just wanted to draw figures.
#include <bits/stdc++.h> #define rep(i, n) for (int i = 0; i < (n); i++) #define repr(i, n) for (int i = (n) - 1; i >= 0; i--) using namespace std; using ll = long long; constexpr int MOD = 998244853; void add(int &x, int y) { x += y; x -= (x >= MOD) * MOD; } int main() { cin.tie(nullptr); ios::sync_with_stdio(false); int n, m; cin >> n >> m; vector<int> c(n + m + 1); c[0] = 1; rep(_, n + m) repr(i, n + m) add(c[i + 1], c[i]); int ans = 0; for (int i = 1; i <= n; i++) add(ans, n - m >= i ? c[n] : c[m + i]); cout << ans << endl; }
\documentclass[dvipdfmx,margin=2mm]{standalone} \usepackage{tikz} \usetikzlibrary{decorations.markings} \begin{document} % https://ipfs-sec.stackexchange.cloudflare-ipfs.com/tex/A/question/39278.html \tikzset{->-/.style={decoration={ markings, mark=at position #1 with {\arrow{latex}}},postaction={decorate}}} \begin{tikzpicture} \node [anchor=west] at (0,5) {$n=4,m=3$}; \draw[thick,-latex] (0,0) -- (7,0); \foreach \i in {0,1,2,3} \draw[dashed] (\i,-\i) -- (\i+4,-\i+4); \foreach \i in {0,1,2,3,4} \draw[dashed] (\i,\i) -- (\i+3,\i-3); \draw [thick] (0,2) -- (7,2); \node at (8,2) {$k=2$}; \foreach \a/\b/\c/\d in {0/0/1/1, 1/1/2/2, 2/2/3/3, 3/3/4/2, 4/2/5/1, 5/1/6/2, 6/2/7/1} \draw[thick,->-=.5] (\a,\b) -- (\c,\d); \node[inner sep=0,minimum size=1mm,fill=black,circle] at (7,1) {}; \end{tikzpicture} \end{document}
\documentclass[dvipdfmx,margin=2mm]{standalone} \usepackage{tikz} \usetikzlibrary{decorations.markings} \begin{document} % https://ipfs-sec.stackexchange.cloudflare-ipfs.com/tex/A/question/39278.html \tikzset{->-/.style={decoration={ markings, mark=at position #1 with {\arrow{latex}}},postaction={decorate}}} \begin{tikzpicture} \node [anchor=west] at (0,5) {$n=4,m=3$}; \draw[thick,-latex] (0,0) -- (7,0); \foreach \i in {0,1,2} \draw[dashed] (\i,-\i) -- (\i+5,-\i+5); \foreach \i in {0,1,2,3,4,5} \draw[dashed] (\i,\i) -- (\i+2,\i-2); \draw [thick] (0,2) -- (7,2); \foreach \a/\b/\c/\d in {0/0/1/1, 1/1/2/2, 2/2/3/1, 3/1/4/2, 4/2/5/3, 5/3/6/2, 6/2/7/3} \draw[thick,->-=.5] (\a,\b) -- (\c,\d); \draw[thick,-latex] (7,1) to[bend right] node[anchor=west] {鏡} (7,3); \node[inner sep=0,minimum size=1mm,fill=black,circle] at (7,1) {}; \node[inner sep=0,minimum size=1mm,fill=black,circle] at (7,3) {}; \end{tikzpicture} \end{document}