Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its depth = 3.

程式碼:

// 104. Maximum Depth of Binary Tree
// https://leetcode.com/problems/maximum-depth-of-binary-tree/description/
// 時間複雜度: O(n), n是樹中的節點個數
// 空間複雜度: O(h), h是樹的高度
class Solution {

    // Definition for a binary tree node.
    public class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int x) { val = x; }
    }

    public int maxDepth(TreeNode root) {

        if(root == null)
            return 0;

        return 1 + Math.max(maxDepth(root.left), maxDepth(root.right));
    }
}