104. Maximum Depth of Binary Tree

Given the root of a binary tree, return its maximum depth.

A binary tree's maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: 3

Example 2:

Input: root = [1,null,2]
Output: 2

Example 3:

Input: root = []
Output: 0

Constraints:

• The number of nodes in the tree is in the range [0, 104].
• -100 <= Node.val <= 100

直接递归

class Solution {
public:
    int maxDepth(TreeNode* root) {
        if(!root) return 0;
        else if(!root->left&&!root->right) return 1;
        else return 1+max(maxDepth(root->right),maxDepth(root->left));
    }
};

Runtime: 8 ms, faster than 64.32% of C++ online submissions for Maximum Depth of Binary Tree.

Memory Usage: 18.8 MB, less than 86.84% of C++ online submissions for Maximum Depth of Binary Tree.

class Solution {
public:
    int maxDepth(TreeNode* root) {
        if(!root) return 0;
        else if(!root->left&&!root->right) return 1; //这一步多余
        else return 1+max(maxDepth(root->right),maxDepth(root->left));
    }
};

Runtime: 4 ms, faster than 92.07% of C++ online submissions for Maximum Depth of Binary Tree.

Memory Usage: 18.8 MB, less than 49.92% of C++ online submissions for Maximum Depth of Binary Tree.

也可改写成三元运算符的形式

class Solution {
public:
    int maxDepth(TreeNode* root) {
        return root==NULL?0:1+max(maxDepth(root->left),maxDepth(root->right));
    }
};