[Swift]LeetCode1120. 子树的最大平均值 | Maximum Average Subtree

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Given the root of a binary tree, find the maximum average value of any subtree of that tree.

(A subtree of a tree is any node of that tree plus all its descendants. The average value of a tree is the sum of its values, divided by the number of nodes.) 

Example 1:

Input: [5,6,1]
Output: 6.00000
Explanation: 
For the node with value = 5 we have and average of (5 + 6 + 1) / 3 = 4.
For the node with value = 6 we have and average of 6 / 1 = 6.
For the node with value = 1 we have and average of 1 / 1 = 1.
So the answer is 6 which is the maximum.

Note:

1. The number of nodes in the tree is between 1 and 5000.
2. Each node will have a value between 0 and 100000.
3. Answers will be accepted as correct if they are within 10^-5 of the correct answer.

---

给你一棵二叉树的根节点 root，找出这棵树的 每一棵 子树的 平均值 中的 最大 值。

子树是树中的任意节点和它的所有后代构成的集合。

树的平均值是树中节点值的总和除以节点数。 

示例：

输入：[5,6,1]
输出：6.00000
解释： 
以 value = 5 的节点作为子树的根节点，得到的平均值为 (5 + 6 + 1) / 3 = 4。
以 value = 6 的节点作为子树的根节点，得到的平均值为 6 / 1 = 6。
以 value = 1 的节点作为子树的根节点，得到的平均值为 1 / 1 = 1。
所以答案取最大值 6。 

提示：

1. 树中的节点数介于 1 到 5000之间。
2. 每个节点的值介于 0 到 100000 之间。
3. 如果结果与标准答案的误差不超过 10^-5，那么该结果将被视为正确答案。

---

88ms

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public var val: Int
 *     public var left: TreeNode?
 *     public var right: TreeNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.left = nil
 *         self.right = nil
 *     }
 * }
 */
class Solution {
    var ret:Double = 0
    func maximumAverageSubtree(_ root: TreeNode?) -> Double {
        if root == nil {return ret}
        dfs(root)
        return ret        
    }
    
    func dfs(_ root: TreeNode?) -> (Int,Int)
    {
        var cal:Int = 1
        var sum:Int = root!.val
        if root!.left != nil
        {
            var test:(Int,Int) = dfs(root!.left)
            cal += test.0
            sum += test.1
        }
        if root!.right != nil
        {
            var test:(Int,Int) = dfs(root!.right)
            cal += test.0
            sum += test.1
        }
        let temp:Double = Double(sum) / Double(cal)
        if ret < temp
        {
            ret = temp
        }
        return (cal, sum)
    }
}