# Leetcode 104. Maximum Depth of Binary Tree (C語言)
- 題目
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.
- 範例

Example:

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

    3
   / \
  9  20
    /  \
   15   7
return its depth = 3. 

int maxDepth(struct TreeNode* root){
    if(!root)return 0;
    return 1+fmax(maxDepth(root->left),maxDepth(root->right));
}

思路:由下往上加回去，如果該點空則回傳0否則回傳1+左右子樹之樹高較高者。