1448. Count Good Nodes in Binary Tree

Medium

Given a binary tree root, a node X in the tree is named good if in the path from root to X there are no nodes with a value greater than X.

Return the number of good nodes in the binary tree.

Example 1:

Input: root = [3,1,4,3,null,1,5]
Output: 4
Explanation: Nodes in blue are good.
Root Node (3) is always a good node.
Node 4 -> (3,4) is the maximum value in the path starting from the root.
Node 5 -> (3,4,5) is the maximum value in the path
Node 3 -> (3,1,3) is the maximum value in the path.

Example 2:

Input: root = [3,3,null,4,2]
Output: 3
Explanation: Node 2 -> (3, 3, 2) is not good, because "3" is higher than it.

Example 3:

Input: root = [1]
Output: 1
Explanation: Root is considered as good.

Constraints:

• The number of nodes in the binary tree is in the range [1, 10^5].

• Each node's value is between [-10^4, 10^4].

解題

記錄到從root到目前節點的最大值即可，如果目前節點的值比最大值大，答案+1

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func goodNodes(root *TreeNode) int {
    return helper(root, -100000)
}

func helper(root *TreeNode, m int) int {
    if root == nil { return 0 }
    
    if root.Val >= m {
        return helper(root.Left, root.Val) + helper(root.Right, root.Val) + 1
    } 

    return helper(root.Left, m) + helper(root.Right, m)
}

func max(a, b int) int {
    if a > b { return a }
    return b
}