leetcode 310. 最小高度树 【时间击败70.67%】 【内存击败89.04%】

数组替代队列，从超时到击败70%，用tree[0]替代new一个新的ArrayList，上升10%

思想是遍历一遍，删除度为1的节点，答案只可能为1或2

public List<Integer> findMinHeightTrees(int n, int[][] edges) {
        ArrayList<Integer>[] tree = new ArrayList[n];
        for (int i = 0; i < n; i++) tree[i] = new ArrayList<>();
        for (int i = 0; i < edges.length; i++) {
            int a = edges[i][0], b = edges[i][1];
            tree[a].add(b);
            tree[b].add(a);
        }
        int[] valid = new int[n];
        Arrays.fill(valid, 1);
        int[] q = new int[n];
        int len = n;
        while (len > 2) {
            q[0] = 0;
            for (int i = 0; i < n; i++) if (tree[i].size() == 1) q[++q[0]] = i;
            len -= q[0];
            for (int j = 1; j <= q[0]; j++) {
                int a = q[j], b = tree[a].get(0);
                tree[a].clear();
                for (int i = 0; i < tree[b].size(); i++) {
                    if (tree[b].get(i) == a) {
                        tree[b].remove(i);
                        break;
                    }
                }
                valid[a] = 0;
            }
        }
        tree[0].clear();
        for (int i = 0; i < n; i++) if (valid[i] == 1) tree[0].add(i);
        return tree[0];
    }