1. 算法思想

图是树的升级版。图通常分为有向（directed）或无向（undirected），有循环（cyclic）或无循环（acyclic），所有节点相连（connected）或不相连（disconnected）。

• 树即是一个相连的无向无环图。
• 有向无环图（Directed Acyclic Graph，DAG）。

图通常有两种表示方法。假设图中一共有n 个节点、m 条边。

• 邻接矩阵（adjacency matrix）：建立一个n× n 的矩阵G，如果第i 个节点连向第j 个节点，则G[i][j] = 1，反之为0；如果图是无向的，则这个矩阵一定是对称矩阵，即G[i][j] = G[j][i]。
• 邻接链表（adjacency list）：建立一个大小为n 的数组，每个位置i 储存一个数组或者链表，表示第i 个节点连向的其它节点。邻接矩阵空间开销比邻接链表大，但是邻接链表不支持快速查找i 和j 是否相连，因此两种表示方法可以根据题目需要适当选择。除此之外，我们也可以直接用一个m × 2 的矩阵储存所有的边。

二分图算法也称为染色法，是一种广度优先搜索。如果可以用两种颜色对图中的节点进行着
色，并且保证相邻的节点颜色不同，那么图为二分。

2. 常见题型

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