[LeetCode]Is Graph Bipartite?

题目描述：

LeetCode 785. Is Graph Bipartite?

Given a graph, return true if and only if it is bipartite.

Recall that a graph is bipartite if we can split it's set of nodes into two independent subsets A and B such that every edge in the graph has one node in A and another node in B.

The graph is given in the following form: graph[i] is a list of indexes j for which the edge between nodes i and j exists.  Each node is an integer between 0 and graph.length - 1.  There are no self edges or parallel edges: graph[i] does not contain i, and it doesn't contain any element twice.

Example 1:Input: [[1,3], [0,2], [1,3], [0,2]]Output: trueExplanation:
The graph looks like this:
0----1
|    |
|    |
3----2
We can divide the vertices into two groups: {0, 2} and {1, 3}.

Example 2:Input: [[1,2,3], [0,2], [0,1,3], [0,2]]Output: falseExplanation:
The graph looks like this:
0----1
| \  |
|  \ |
3----2
We cannot find a way to divide the set of nodes into two independent ubsets.

Note:

• graph will have length in range [1, 100].
• graph[i] will contain integers in range [0, graph.length - 1].
• graph[i] will not contain i or duplicate values.

题目大意：

以邻接表形式给定无向图graph，判断是否为二分图

解题思路：

将邻接表转化为邻接矩阵

然后判断每一组顶点集互相之间是否有边相连，若存在则返回False

返回True

Python代码：

class Solution(object):
    def isBipartite(self, graph):
        """
        :type graph: List[List[int]]
        :rtype: bool
        """
        edges = collections.defaultdict(set)
        for idx, points in enumerate(graph):
            for p in points: edges[idx].add(p)
        for points in graph:
            for x in range(len(points)):
                for y in range(x + 1, len(points)):
                    if points[y] in edges[points[x]]:
                        return False
        return True