题目：

题解：

func findMode(root *TreeNode) (answer []int) {
    var base, count, maxCount int
    update := func(x int) {
        if x == base {
            count++
        } else {
            base, count = x, 1
        }
        if count == maxCount {
            answer = append(answer, base)
        } else if count > maxCount {
            maxCount = count
            answer = []int{base}
        }
    }
    cur := root
    for cur != nil {
        if cur.Left == nil {
            update(cur.Val)
            cur = cur.Right
            continue
        }
        pre := cur.Left
        for pre.Right != nil && pre.Right != cur {
            pre = pre.Right
        }
        if pre.Right == nil {
            pre.Right = cur
            cur = cur.Left
        } else {
            pre.Right = nil
            update(cur.Val)
            cur = cur.Right
        }
    }
    return
}