从零开始的LC刷题(23): Maximum Depth of Binary Tree 二叉树深度

原题：

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.

求二叉树深度只要dfs就好了，每个节点求左右子树最大值并返回最大值+1，结果：

Success

Runtime: 8 ms, faster than 98.23% of C++ online submissions for Maximum Depth of Binary Tree.

Memory Usage: 19.1 MB, less than 93.50% of C++ online submissions for Maximum Depth of Binary Tree.

代码：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int maxDepth(TreeNode* root) {
        if (root==NULL){return 0;}
        int l=maxDepth(root->left);
        int r=maxDepth(root->right);
        if (l>=r){return l+1;}
        else return r+1;
    }
};