310. Minimum Height Trees

class Solution(object):
    def findMinHeightTrees(self, n, edges):
        """
        :type n: int
        :type edges: List[List[int]]
        :rtype: List[int]
        """
        #a graph has maxmimum 2 minimum height tree 
        #peel away the leaves one layer at a time until there are two or fewer nodes 
        if n==1: return [0]
        adj=[set() for _ in range(n)]
        for i,j in edges:
            adj[i].add(j)
            adj[j].add(i)
        leaves=[i for i in xrange(n) if len(adj[i])==1]
        while n>2:
            n-=len(leaves)
            new_leaves=[]
            for leave in leaves:
                j=adj[leave].pop()
                adj[j].remove(leave) 
                if len(adj[j])==1:new_leaves.append(j)
            leaves=new_leaves
        return leaves