Han Zhu's Study Notes

There are a total of  n  courses you have to take, labeled from  0  to  n - 1 . Some courses may have prerequisites, for example to take course 0 you have to first take course 1, which is expressed as a pair:  [0,1] Given the total number of courses and a list of prerequisite  pairs , return the ordering of courses you should take to finish all courses. There may be multiple correct orders, you just need to return one of them. If it is impossible to finish all courses, return an empty array. For example: 2, [[1,0]] There are a total of 2 courses to take. To take course 1 you should have finished course 0. So the correct course order is  [0,1] 4, [[1,0],[2,0],[3,1],[3,2]] There are a total of 4 courses to take. To take course 3 you should have finished both courses 1 and 2. Both courses 1 and 2 should be taken after you finished course 0. So one correct course order is  [0,1,2,3] . Another correct ordering is [0,2,1,3] . Note:...

There are a total of  n  courses you have to take, labeled from  0  to  n - 1 . Some courses may have prerequisites, for example to take course 0 you have to first take course 1, which is expressed as a pair:  [0,1] Given the total number of courses and a list of prerequisite  pairs , is it possible for you to finish all courses? For example: 2, [[1,0]] There are a total of 2 courses to take. To take course 1 you should have finished course 0. So it is possible. 2, [[1,0],[0,1]] There are a total of 2 courses to take. To take course 1 you should have finished course 0, and to take course 0 you should also have finished course 1. So it is impossible. Note: The input prerequisites is a graph represented by  a list of edges , not adjacency matrices. Read more about  how a graph is represented . You may assume that there are no duplicate edges in the input prerequisites. click to show more hints. Hints: This problem is ...

There is a new alien language which uses the latin alphabet. However, the order among letters are unknown to you. You receive a list of  non-empty  words from the dictionary, where  words are sorted lexicographically by the rules of this new language . Derive the order of letters in this language. Example 1: Given the following words in dictionary, [ "wrt", "wrf", "er", "ett", "rftt" ] The correct order is:  "wertf" . Example 2: Given the following words in dictionary, [ "z", "x" ] The correct order is:  "zx" . Example 3: Given the following words in dictionary, [ "z", "x", "z" ] The order is invalid, so return  "" . Note: You may assume all letters are in lowercase. You may assume that if a is a prefix of b, then a must appear before b in the given dictionary. If the order is invalid, return an empty string...