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??TreeNode??

public class TreeNode {    int val;    TreeNode left;    TreeNode right;    TreeNode(int x) {        val = x;    }}

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public class Solution {    public int maxDepth(TreeNode root) {        return find(root, 0);    }    private int find(TreeNode root, int num) {        if (root == null) {            return num;        }        return Math.max(find(root.left, num + 1), find(root.right, num + 1));    }}

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