Minimum Height Trees

https://www.lintcode.com/problem/minimum-height-trees/description

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class Solution {
    /**
     * @param n: n nodes labeled from 0 to n - 1
     * @param edges: a undirected graph
     * @return: a list of all the MHTs root labels
     */
    public List<Integer> findMinHeightTrees(int n, int[][] edges) {
        // Wirte your code here
//        叶子节点肯定比中间节点的高
        if (n == 1) return Collections.singletonList(0);
        List<Integer> leaves = new ArrayList<>();
        List<Set<Integer>> adj = new ArrayList<>(n);
        for (int i = 0; i < n; ++i) adj.add(new HashSet<>());
        for (int[] edge : edges) {
            adj.get(edge[0]).add(edge[1]);
            adj.get(edge[1]).add(edge[0]);
        }
        for (int i = 0; i < n; ++i) {
            if (adj.get(i).size() == 1) leaves.add(i);
        }
        while (n > 2) {
            n -= leaves.size();
            List<Integer> newLeaves = new ArrayList<>();
            for (int i : leaves) {
                int t = adj.get(i).iterator().next();
                adj.get(t).remove(i);
                if (adj.get(t).size() == 1) newLeaves.add(t);
            }
            leaves = newLeaves;
        }
        return leaves;
    }

}