A directed graph (or digraph) is a set of vertices and a collection of directed edges that each connects an ordered pair of vertices. We say that a directed edge points from the first vertex in the pair and points to the second vertex in the pair¹.

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Binary search is a search algorithm. The fundamental is that it reduces the searching range into half each time by comparing the middle element to the target. Normal binary search takes O(logn) comparisons, when n is the number of input. Space actually depends on the implementation. If not using recursion, there would be no extra space used in a regular binary search. Therefore, it normally is O(1).

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A graph is formed by the non-empty set of vertex and set of edge. Use G <E, V> to represent a graph, G, and the sets of it’s vertex, V, and edge, E.

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React, a JavaScript library for building user interfaces.¹

观察者模式: 定义对象间的一种一对多的依赖关系, 当一个对象的状态发生改变时, 所有依赖于它的对象都得到通知并自动更新¹.

There is always a corresponding wrapper class for each primitive type in Java, and all wrapper classes are IMMUTABLE. Then what is a wrapper class for?

• Can be used in Java generic, for example, the obect in List<Object> are wrapper class of primitive type.
• Can be used to define and add member function.
• Can be represented by null if not exists.

E.g. char --> Character

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Singleton status is wether exists, and it only has one instance if it exists.

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─=≡Σ((( つ•̀ω•́)つ BS🌲

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─=≡Σ((( つ•̀ω•́)つ Traverse 🌲

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A directed acyclic graph (DAG!) is a directed graph that contains no cycles. A rooted tree is a special kind of DAG and a DAG is a special kind of directed graph. For example, a DAG may be used to represent common subexpressions in an optimising compiler.[1]

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