1448. Count Good Nodes in Binary Tree

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].

class Solution {
    public int goodNodes(TreeNode root) {
        return help(root, -10001);
    }
    public int help(TreeNode root, int num){
        if(root == null) return 0;
        int curmax = Math.max(root.val, num);
        return (root.val >= num ? 1: 0) + help(root.left, curmax) + help(root.right, curmax);
    }
}

preorder遍历，找出遍历至今的当前最大curmax，和root.val比较，然后继续preorder