104. Maximum Depth of Binary Tree

原题链接：https://leetcode.com/problems/maximum-depth-of-binary-tree/description/
这道题目级别为“Easy”，也确实是简单！
不废话，直接使用递归实现深度优先搜索即可：

/**
 * Created by clearbug on 2018/2/26.
 */
public class Solution {

    static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;

        public TreeNode(int val) {
            this.val = val;
        }
    }

    public static void main(String[] args) {
        TreeNode root = new TreeNode(1);

        TreeNode rootLeft = new TreeNode(2);
        TreeNode rootRight = new TreeNode(3);
        root.left = rootLeft;
        root.right = rootRight;

        TreeNode leftLeft = new TreeNode(3);
        TreeNode leftRight = null;
        rootLeft.left = leftLeft;
        rootLeft.right = leftRight;

        TreeNode rightLeft = new TreeNode(2);
        TreeNode rightRight = null;
        rootRight.left = rightLeft;
        rootRight.right = rightRight;

        Solution s = new Solution();
        System.out.println(s.maxDepth(root));
    }

    public int maxDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }
        return dfs(root, 1);
    }

    public int dfs(TreeNode node, int currentDepth) {

        int leftDepth = currentDepth, rightDepth = currentDepth;
        if (node.left != null) {
            leftDepth = dfs(node.left, currentDepth + 1);
        }
        if (node.right != null) {
            rightDepth = dfs(node.right, currentDepth + 1);
        }

        return leftDepth > rightDepth ?
                (leftDepth > currentDepth ? leftDepth : currentDepth) :
                (rightDepth > currentDepth ? rightDepth : currentDepth);
    }
}