方法一：
/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ class Solution { List<Integer> answer = new ArrayList<Integer>(); int base, count, maxCount; public int[] findMode(TreeNode root) { dfs(root); int[] mode = new int[answer.size()]; for (int i = 0; i < answer.size(); ++i) { mode[i] = answer.get(i); } return mode; } public void dfs(TreeNode o) { if (o == null) { return; } dfs(o.left); update(o.val); dfs(o.right); } public void update(int x) { if (x == base) { ++count; } else { count = 1; base = x; } if (count == maxCount) { answer.add(base); } if (count > maxCount) { maxCount = count; answer.clear(); answer.add(base); } } }