{"trustable":false,"prependHtml":"\u003cscript\u003e window.katexOptions \u003d { disable: true }; \u003c/script\u003e\n\u003cscript type\u003d\"text/x-mathjax-config\"\u003e\n MathJax.Hub.Config({\n tex2jax: {\n inlineMath: [[\u0027$$$\u0027,\u0027$$$\u0027], [\u0027$\u0027,\u0027$\u0027]],\n displayMath: [[\u0027$$$$$$\u0027,\u0027$$$$$$\u0027], [\u0027$$\u0027,\u0027$$\u0027]]\n }\n });\n\u003c/script\u003e\n\u003cscript async src\u003d\"https://mathjax.codeforces.org/MathJax.js?config\u003dTeX-AMS-MML_HTMLorMML\" type\u003d\"text/javascript\"\u003e\u003c/script\u003e","sections":[{"title":"","value":{"format":"HTML","content":"\u003cscript type\u003d\u0027text/x-mathjax-config\u0027\u003eMathJax.Hub.Config({tex2jax: { inlineMath: [[\u0027$\u0027,\u0027$\u0027]] } }); \u003c/script\u003e\n\u003cscript type\u003d\u0027text/javascript\u0027 src\u003d\u0027https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.1/MathJax.js?config\u003dTeX-AMS-MML_HTMLorMML\u0027\u003e\u003c/script\u003e\n\u003cscript type\u003d\u0027text/javascript\u0027\u003esetTimeout(function(){MathJax.Hub.Queue([\u0027Typeset\u0027, MathJax.Hub, \u0027left_view\u0027]);}, 2000);\u003c/script\u003e\n\u003cdiv class\u003d\"panel_content\"\u003e\n 我们都知道一个师傅可以有很多徒弟,一个徒弟也可以有很多师傅,这是合法的。然而,一些师徒持有非法关系。以a和b为例,a是b的师傅,同时b也是a的师傅,这是违法的!为了避免这种情况,请帮助我们判断他们的关系是否合法。\n\u003cbr\u003e请注意,师徒关系是可传递的。这意味着如果A是B的师傅,B是C的师傅,那么A也是C的师傅。\n\u003c/div\u003e"}},{"title":"Input","value":{"format":"HTML","content":"输入包含多组测试样例。对于每组样例,第一行包含两个整数, N (人数) and M (关系数)(2 \u0026lt;\u003d N, M \u0026lt;\u003d 100). 接下来 M 行 ,每行包含一对 (x, y) ,这表示x是y的师傅,y是x的徒弟。当输入N \u003d 0时结束. \n\u003cbr\u003e为了让题目简单,我们给每个人一个编号(0, 1, 2,..., N-1). 我们使用编号代替他们的名字"}},{"title":"Output","value":{"format":"HTML","content":"对每个样例来说,如果关系合法输出YES,否则输出NO\n\u003cbr\u003eIf it is legal, output \"YES\", otherwise \"NO\"."}},{"title":"Sample Input","value":{"format":"HTML","content":"\u003cpre\u003e3 2\n0 1\n1 2\n2 2\n0 1\n1 0\n0 0\u003c/pre\u003e"}},{"title":"Sample Output","value":{"format":"HTML","content":"\u003cpre\u003eYES\nNO\u003c/pre\u003e"}}]}