遍历二叉树,用map记录sum出现的次数,每一个新的节点都统计一次。

遍历完就统计map中出现最多的sum

Map<Integer,Integer> map = new HashMap<>();
public int[] findFrequentTreeSum(TreeNode root) {
helper(root);
int max = 0;
List<Integer> list = new ArrayList<>();
for (int key : map.keySet()) {
int v = map.get(key);
if (v>max)
{
list.clear();
list.add(key);
max = v;
}
else if (v==max) list.add(key);
}
int[] res = new int[list.size()];
for (int i = 0; i < res.length; i++) {
res[i] = list.get(i);
}
return res;
}
public int helper(TreeNode root)
{
if (root==null) return 0;
int cur = root.val;
cur+= helper(root.left);
cur+=helper(root.right);
map.put(cur,map.getOrDefault(cur,0)+1);
return cur;
}

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