题目：

题解：

func findFrequentTreeSum(root *TreeNode) (ans []int) {
    cnt := map[int]int{}
    maxCnt := 0
    var dfs func(*TreeNode) int
    dfs = func(node *TreeNode) int {
        if node == nil {
            return 0
        }
        sum := node.Val + dfs(node.Left) + dfs(node.Right)
        cnt[sum]++
        if cnt[sum] > maxCnt {
            maxCnt = cnt[sum]
        }
        return sum
    }
    dfs(root)

    for s, c := range cnt {
        if c == maxCnt {
            ans = append(ans, s)
        }
    }
    return
}