刷题打卡

从3/4开始每天5道 （从easy开始）
刷2个月刷完leetcode 200题
只用c++
然后计划歇一阵子，暑假接着干，每天4-5道题
刷2月再刷一遍那200-300题
最后总结一下题目
加油～～

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今天刷的题

find a cycle in a directed/ undirected graph
Directed Graph：

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DFS
3 kinds of nodes:
black: already explored, no cycle
grey: under exploration, on one path
white: not explored

turn grey to black: all of its children has visited, and there is no  cycle

if the node next to visit is grey: found a cycle

if the node next to visit is black: no need to visit, since all of its children has been explored, and no cycle found(all black)

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topological sort

Union Find

Undirected graph：

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2 kinds of nodes black and white(don’t need grey) like directed graph

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topological sort
求图中所有点的度
删除度<=1的node，和相邻的边，
重复2直到无未删除的点
如果存在未删除的点，则有环，否则无环

DFS~~~
Similar to Maze and 8-Queen Problem

Reference:
GranYang


Leetcode Combination Sum & Combination Sum II:
Recursion & Backtraking

return all vals：mostly use Recrusion to solve  it.
Like CombinationSum 2
PathSum 2
Subset 2
Permutations 1,2
Combnations